From ‘The Duties of Brotherhood’ by Al-Ghazali*:
The third duty concerns the tongue, which should sometimes be silent and at other times speak out.
As for silence, the tongue should not mention a brother’s faults in his absence or in his presence. Rather should you feign ignorance. You should not contradict him when he talks, nor dispute nor argue with him. You should not pry and quiz him about his affairs. On seeing him in the street or about some business, you should not start a conversation about the object of your coming and going, nor ask him about his, for perhaps it will be troublesome to him to discuss it, or he may have to lie about it.
Keep silent also about the secrets he confides in you, and on no account divulge them to a third party–not even to his closest friends. Do not reveal anything about them, not even after separation and estrangement, for to do so would be meanness of character and impurity of the Inner.
Keep silent form criticism of his dear ones, his family and his children; also from relating other people’s criticism of him, for it is your informant who directly abuses you.
Anas said that God’s Messenger** (Goid bless him and give him Peace!) never faced anyone with something displeasing to him, for the hurt comes immediately from the informant and only indirectly from the original speaker.
Of course you should not hide any praise you may hear, for the pleasure in it is received directly from the conveyer of the compliment as well as indirectly from the original source. Concealment here would mean envy.
In short, you should keep silent about any speech unpleasant to him in general and in particular–unless obliged to speak out to promote good and to prevent evil, and even then only if you can find no valid excuse for saying nothing. In such cases you need not worry about his disapproval, since what you do is beneficial to him when rightly understood, even if it looks bad at first sight.
*Abu Hamid Al-Ghazali (1058-1111 CE) was a Persian theologian and mystic whose work had a tremendous effect on the intellectual and spiritual development of the world. Among other aspects of his teaching, his clear reasoning helped to reconcile orthodox religion with the unorthodox, mystical vitality of the Sufis. The present text is taken from ‘On the Duties of Brotherhood,’ part of a work called “The Revival of Religious Sciences.” Al-Ghazali gives eight duties: material assistance, personal aid, holding one’s tongue, speaking out, forgiveness, prayer, loyalty and sincerity, and informality.
**The Prophet Muhammad.
from On the Duties of Brotherhood
tr. Muhtar Holland