Kitāb al-Muʾmin
Hadith 2727 - عن ابن حمران قال: سمعته يقول: ما مِن مؤمنٍ يمرُّ به أربعون ليلةً إلاّ وقد يُذكر بشئٍ يؤجر عليه، أدناه هَمٌّ لا يدري من أين هو.
27. It has been narrated that Ibn Hamran said: I heard him (i.e. al-Sadiq (a.s)) say: No forty nights pass for a believer without something befalling him for which he is rewarded - the least of which is an anxiety whose cause he does not understand.