(70) باب صَوْمِ النَّبِيِّ صلى الله عليه وسلم - بِأَبِي هُوَ وَأُمِّي - وَذِكْرِ اخْتِلاَفِ النَّاقِلِينَ لِلْخَبَرِ فِي ذَلِكَ
It was narrated that Hafsah said:
"When the Messenger of Allah would lie down, he would place his right hand under his right cheek, and he used to fast on Mondays and Thursday."
Reference: Sunan an-Nasa'i 2367
In-book reference: Book 22, Hadith 278
English translation: Vol. 3, Book 22, Hadith 2369