Al-Isra (The Ascension) Sura 17: Verse 53
Tell My servants (O Muhammad), to always say that which is best. Verily it is Satan who sows discord among men. Satan indeed is an open enemy to man.
In their discussions with the unbelievers and polytheists, in fact with all opponents of their faith, Muslims should refrain from losing their temper. Additionally, they should not resort to exaggerated and extremist statements. Even in the face of provocation from their opponents, Muslims should not utter even a word that is contrary to the truth; nor should they lose their temper at the vulgarities which are flung at them by their opponents, nor should they be provoked to the point of paying back their opponents in the latter's own coins. Instead, they should keep their composure and say only that which is balanced and true, and is in keeping with the grace and dignity of the faith which they seek to uphold.
The Quran tells believers that whenever in the course of their discussion on contentious issues with unbelievers, they feel overly provoked and overwhelmed with rage, they should immediately realize that such reactions are instigated by Satan who is keen to hurt the cause of their faith. Satan will certainly try to prompt the believers to give up, as their opponents had done, all efforts of reform, and to become entangled in futile wrangling and strife. For it is Satan's aim that all mankind remain perpetually enmeshed in such controversies.
Compiled From:"Towards Understanding the Quran" - Abul Ala Mawdudi, Vol. 5, p. 51