Kitāb al-Muʾmin
Hadith 197197 - وعن أبي جعفر عليه السلام أنه قال: من اغتيب عنده أخوه المؤمن فلم ينصره، ولم يدفع عنه، وهو يقدر على نصرته وعونه فضحه الله عز وجل في الدنيا والآخرة.
197. It has been narrated that Abu Ja'far (a.s) said: If one's brother-in-faith is backbitten in his presence while he does neither aid nor defend him when he is able to do so, Allah will disgrace him in the world and the hereafter.