The Time for Night Prayer
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 66
وَ رَوَى عِيصُ بْنُ اَلْقَاسِمِ عَنْ أَبِي عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ أَنَّهُ قَالَ: «إِذَا غَلَبَ اَلرَّجُلَ اَلنَّوْمُ وَ هُوَ فِي اَلصَّلاَةِ فَلْيَضَعْ رَأْسَهُ فَلْيَنَمْ فَإِنِّي أَتَخَوَّفُ عَلَيْهِ إِنْ أَرَادَ أَنْ يَقُولَ اَللَّهُمَّ أَدْخِلْنِي اَلْجَنَّةَ أَنْ يَقُولَ اَللَّهُمَّ أَدْخِلْنِي اَلنَّارَ.
Ays ibn al-Qasim reported from Imam Jafar ibn Muhammad Al-Sadiq عليه السلام who said: "If a man is overcome by sleep while he is in prayer, let him put his head down and sleep, for I fear that if he wishes to say, 'O’ Allah سبحانه وتعالى, admit me to Paradise,' he might instead say, 'O’ Allah سبحانه وتعالى, admit me to the Fire.'"
- Ṣaḥīḥ al-Faqīh, p. 70