What Nullifies Ablution
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 16
وَ سَأَلَهُ سَمَاعَةُ بْنُ مِهْرَانَ: عَنِ اَلرَّجُلِ يُخْفِقُ رَأْسَهُ وَ هُوَ فِي اَلصَّلاَةِ قَائِماً أَوْ رَاكِعاً قَالَ «لَيْسَ عَلَيْهِ وُضُوءٌ.
Sama'a ibn Mehran asked him about a man who nods off while standing or bowing in prayer. Imam عليه السلام said: 'He does not need to perform ablution.'.
- Ṣaḥīḥ al-Faqīh, p. 24