Sunan an-Nasai 17108
In-book no 2372أَخْبَرَنَا 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."