كتاب الحيض
3
The Book of Menstruation
(33)
Chapter: Evidence that sleeping while sitting does not invalidate wudu’
(33)
باب الدَّلِيلِ عَلَى أَنَّ نَوْمَ الْجَالِسِ لاَ يَنْقُضُ الْوُضُوءَ ‏

Qatida reported:

I heard Anas as saying that the Companion of the Messenger of Allah (ﷺ) dozed off and then offered prayer and did not perform ablution. He (the narrator) said: I asked him if he had actually heard it from Anas. He said: By Allah. yes.
وَحَدَّثَنِي يَحْيَى بْنُ حَبِيبٍ الْحَارِثِيُّ، حَدَّثَنَا خَالِدٌ، - وَهُوَ ابْنُ الْحَارِثِ - حَدَّثَنَا شُعْبَةُ، عَنْ قَتَادَةَ، قَالَ سَمِعْتُ أَنَسًا، يَقُولُ كَانَ أَصْحَابُ رَسُولِ اللَّهِ صلى الله عليه وسلم يَنَامُونَ ثُمَّ يُصَلُّونَ وَلاَ يَتَوَضَّئُونَ قَالَ قُلْتُ سَمِعْتَهُ مِنْ أَنَسٍ قَالَ إِي وَاللَّهِ ‏.‏
Reference : Sahih Muslim 376 c
In-book reference : Book 3, Hadith 156
USC-MSA web (English) reference : Book 3, Hadith 733
  (deprecated numbering scheme)