Shaykh 'Abdul-'Azîz bin Bâz - rahimahullâh - said:
“The etiquettes, manners and characteristics necessary for the du'ât
(callers to Islâm) has been duly explained by Allâh - the Mighty and Majestic - in many âyât and in many different places in the Noble Qur‘ân. And from those necessary etiquettes are:
So it is obligatory upon the dâ'î (caller) to have ikhlâs (sincerity and purity of intentions and actions) for Allâh - the Mighty and Majestic - neither desiring to showoff, nor desiring reputation, nor desiring the praises and the accolades of the people. Rather, the da'î should only call to Allâh seeking the Face of Allâh, as He - the One free from all imperfections - says:
“Say: This is my path, I do call to upon knowledge.”And Allâh - the Mighty and Majestic - said:
“And who is better in speech than one who calls to Allâh.”Thus it is a must to have ikhlâs and to call only to Allâh - the
Mighty and Majestic - and this is the most important ettiquette and the greatest quality; that you seek from your da'wah(call) the Face of Allâh and the Home of the Hereafter.
To call the people to upon 'ilm (knowledge), and not ignorance:
“Say: This is my path, I do call to Allâh upon knowledge.”So knowledge is an essential obligation in calling to Allâh. So beware of calling to Allâh based upon ignorance, and beware of speaking without due knowledge. Indeed, ignorance destroys, it does not build; and it causes corruption, not reformation and correction. So - O worshipper of Allâh - fear Allâh and beware of speaking about Allâh without due knowledge, and do not call to anything except after knowledge of it and having insight into what Allâh and His Messenger sallallâhu 'alayhi wa sallam have said. So it is upon the student of knowledge and the dâ'î to have knowledge of, and insight into that which they call to, knowing its proofs and evidences.
Thirdly: Mildness and Gentleness
From the akhlâq that it is necessary to have - O dâ'î - is to be mild and forbearing in your da'wah, and being gentle and patient in it, as were all the Messengers 'alayhimus-salâtu was-salâm. Beware of being hasty, harsh and strict in your da'wah, rather be patient, mild and gentle. In this regard, some of the proofs have already preceeded, such as the saying of Allâh - the Mighty and
“Call to the way of your Lord with wisdom and beautiful admonition, and argue with them in ways that are best.”And His - the Most Perfect's - saying:
“And by the Mercy of Allâh you were able to deal gently with them. If you had been severe and harsh-hearted they would have broken away from about you.”And His - the Most Majestic's - saying, with regards to Mûsâ and
“So speak to him mildly, perchance he may take admonition, or that he may fear Allâh.”And the Prophet sallallhu 'alayhi wa sallam said in the authentic hadîth: “O Allâh! Whosoever is a guardian over any of the affairs concerning my Ummah and he is gentle with them, then be gentle with him. And whosoever is a guardian over any of the affairs concerning my Ummah and he is harsh with them, then be harsh with him.”
So - O servant of Allâh - it is necessary upon you to be gentle in your da'wah and not to be harsh upon the people. Do not turn the people away from the Dîn due to your harshness, ignorance, or other such behaviour. On the contrary, be mild, forbearing and patient; and be soft and pleasant in speech, so your words may
have an effect upon the heart brother, or that it may have an effect upon the one you are addressing the call to. Then the people will better appreciate your call and invitation. So undue strictness causes people to become distant, not close; and it causes separation, not unity. Therefore, it is a must to be gentle, as the Messenger 'alayhis-salâtu was-salâm said: “Indeed gentleness does not enter into anything except that it beautifies it, is it removed from anything except that it disfigures it.” 
And he 'alayhis-salâtu was-salâm also said: “Whoever is prevented from gentleness, is actually prevented from all good and excellence.” 
Fourthly: Setting an Example
From the necessary or rather obligatory - etiquettes and qualities that a dâ'î must possess is acting in accordance to what he is calling to, and being a righteous example of what is being called to. He should not call to do something and then not do it himself, nor call to leave something, whilst engaging in it himself.
This is the condition of the losers - we seek Allâh's refuge in this! It is those Believers who call to the truth, act upon what they call to the truth and hasten to righteousness and avoid the prohibited, these are the ones who will be successful and who will be rewarded. Allâh - the Most Majestic - says:
“O you who believe! do you say that which you do not do. It is most hateful in the sight of Allâh that you say that which you do not do.”Likewise, Allâh - the One free from all imperfections said, whilst
condemning the Jews for ordering the people with righteousness
whilst forgetting it themselves:
“Do you enjoin righteousness upon the people whilst you yourselves forget to practice it, and you recite the Book? Have you no sense?”And it has been established from the Prophet sallallâhu 'alayhi wa sallam that he said: “A man will be brought on the Day of Judgement and he will be thrown into the HellFire, so that his intestines will come out and he will go around like a donkey goes around the millstone. The people of HellFire will gather around him and say: O soandso! What happened to you? Did you not used to order us with good and prohibit us from evil? He will say: I used to order you with good and not do it myself; and I used to prohibit you from evil and do it myself.” 
This will be the situation of the one who calls to Allâh, ordering the good and prohibiting the evil; whilst acting contrary to one's saying, or saying things contrary to ones actions - we seek refuge in Allâh from this. Therefore, from the most important qualities and one greatest obligations upon the dâ'î is to act upon what he calls to and abstain from that which he prohibits. The dâ'î should have an excellent character and praiseworthy conduct, being patient and inviting to patience. The dâ'î should be sincere in his da'wah and strive in spreading goodness to the people and keeping them away falsehood. At the same time the dâ'î should supplicate for the guidance of others, saying: “O Allâh! Guide him, and grant him the ability to accept the truth.”. So from the excellent manners of the da'î is supplicating, guiding and being patient with the harms that come with this da'wah
When the Prophet 'alayhis-salâtu was-salâm was informed that the tribe of Daws had become disobedient he said: “O Allâh! Guide Daws and bring them.” So the dâ'î should supplicate for guidance and ability to accept the truth for the one he is calling, and he should be patient and encourage patience in this. He should not despair, nor feel hopeless, nor say anything except good. He should not be harsh and strict, nor should he say a word which may cause aversion to the truth.
However, if anyone commits aggression and oppression, then a different treatment is to be to such people, as Allâh - the Most Majestic - says:
“And do not argue with the people of the Book except in a good way, except those who do wrong.”So a wrongdoer, who opposes the da'wah with evil and enmity and seeks to cause harm, is to be dealt with in a different manner. If possible such a person should be imprisoned, or something similar to that - depending upon the nature of his to the da'wah. However, as long as he causes no harm, then it upon you to be patient and selfevaluating and to debate with him in ways that are best. If any personal harm was caused by such a person, then such harm should be borne with patience - as did the Messengers and those who followed them in goodness and righteousness, patiently bear such harms.
I ask Allâh to grant us all the wellbeing and the ability to convey this da'wah in a wise manner, that He corrects our hearts and our actions, and that He grants to us the understanding of the Dîn and firmness upon it making us of those who are guided and guiding others, righteous and teaching others righteousness. Indeed He is the Most Majestic, the Supreme, the Most Generous.”
2. Sûrah Yûsuf 12:108
3. Sûrah Fussilat 41:33
4. Sûrah Yûsuf 12:108
5. Sûrah Nahl 16:125
6. Sûrah Âl-'Imrân 3:159
7. Sûrah Tâ Hâ 20:44
8. Related by Muslim (12/212) from 'Âishah radiallâhu 'anhâ
9. Related by Muslim (16/146) from Âishah radiallâhu 'anhâ
10. Related by Muslim (16/145) from Jâbir ibn Abdullâh
11. Sûrah as-Saff 61:2-3
12. Sûrah alBaqarah 2:44
13. Related alBukhârî (6/331) and Muslim (18/118) from Usâmah ibn Zayd radiallâhu 'anhu
14. Related by al-Bukhârî (6/105) from Abû Hurayrah.
15. Sûrah alAnkabût 29:46