Sunan an-Nasa'i, Hadith 1627 ⓘ
It was narrated that Anas said: "Every time we wanted to see the Messenger of Allah praying at night we saw him, and every time we wanted to see him sleeping, we saw him."
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا يَزِيدُ، قَالَ أَنْبَأَنَا حُمَيْدٌ، عَنْ أَنَسٍ، قَالَ مَا كُنَّا نَشَاءُ أَنْ نَرَى رَسُولَ اللَّهِ صلى الله عليه وسلم فِي اللَّيْلِ مُصَلِّيًا إِلاَّ رَأَيْنَاهُ وَلاَ نَشَاءُ أَنْ نَرَاهُ نَائِمًا إِلاَّ رَأَيْنَاهُ .
also referenced under: Book 20, Hadith 30 (In-Book) & Volume 2, Book 20, Hadith 1628 (DarusSalam)