Sahih al-Bukhari 213
Sahih al-Bukhari · Book 4: Ablutions (Wudu') · Ablution after sleep. And whoever considers it unnecessary to repeat ablution after dozing once or twice or after nodding once in slumber
Narrated Anas:
حَدَّثَنَا أَبُو مَعْمَرٍ ، قَالَ حَدَّثَنَا عَبْدُ الْوَارِثِ ، حَدَّثَنَا أَيُّوبُ ، عَنْ أَبِي قِلاَبَةَ ، عَنْ أَنَسٍ ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " إِذَا نَعَسَ أَحَدُكُمْ فِي الصَّلاَةِ فَلْيَنَمْ حَتَّى يَعْلَمَ مَا يَقْرَأُ " .
The Prophet (ﷺ) said, "If anyone of you feels drowsy while praying, he should sleep till he understands what he is saying (reciting).