The Book of Fasting
22/70. Chapter:
The fast of the Prophet
Sunan an-Nasai
أَخْبَرَنَا الْقَاسِمُ بْنُ زَكَرِيَّا بْنِ دِينَارٍ، قَالَ حَدَّثَنَا حُسَيْنٌ، عَنْ زَائِدَةَ، عَنْ عَاصِمٍ، عَنِ الْمُسَيَّبِ، عَنْ حَفْصَةَ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا أَخَذَ مَضْجَعَهُ جَعَلَ كَفَّهُ الْيُمْنَى تَحْتَ خَدِّهِ الأَيْمَنِ وَكَانَ يَصُومُ الاِثْنَيْنَ وَالْخَمِيسَ .
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 2367In-book reference : Book 22, Hadith 278English translation : Vol. 3, Book 22, Hadith 2369