(53) بَابُ الْوُضُوءِ مِنَ النَّوْمِ وَمَنْ لَمْ يَرَ مِنَ النَّعْسَةِ وَالنَّعْسَتَيْنِ أَوِ الْخَفْقَةِ وُضُوءًا
Narrated Anas:
The Prophet (ﷺ) said, "If anyone of you feels drowsy while praying, he should sleep till he understands what he is saying (reciting).
Reference: Sahih al-Bukhari 213
In-book reference: Book 4, Hadith 79
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 4, Hadith 212