Sunan an-Nasa'i, Book 22, Hadith 278 (In-Book) ⓘ
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."
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 .
also referenced under: Hadith 2367 (Reference) & Volume 3, Book 22, Hadith 2369 (DarusSalam)