The following is taken from The Duties of Brotherhood by Abu Hamid Al-Ghazali, who declared that there are eight duties towards those one considers a member of one’s spiritual family: material assistance, personal aid, holding one’s tongue, speaking out, forgiveness, prayer, loyalty and sincerity, and informality. Some of his thoughts on keeping silent were given in this post, where there is also more information about the author.

Just as brotherhood calls for silence about unpleasant things, so it requires the utterance of favourable things. Indeed, this is more particularly a feature of brotherhood, because anyone satisfied with silence alone might as well seek the fellowship of the People of the Tombs. You wish for brothers so as to benefit by them, not just to escape being hurt by them, and the point of silence is to avoid hurt.

Brotherhood means participating together in joy and sadness.

You should use the tongue to express affection to your brother, and to enquire agreeably about his circumstances. For instance, in asking about some accident that has befallen him, you should show the heart’s concern on his behalf and over his slow recovery. Thus you should indicate by word and deed that you disapprove of all circumstances that are disagreeable to him, and use your tongue to let him know that you share his joy in all conditions that give him pleasure. For brotherhood means participating together in joy and sadness.

The Prophet (God bless him and give him Peace!) said :
If one of you loves his brother, let him know it!

from On the Duties of Brotherhood
al-Ghazali
tr. Muhtar Holland