Whenever the Messenger of Allah ﷺ had prayed the two units of the dawn prayer, he would lie down on his side.
حدثنا عمر بن هشام حدثنا النضر بن شميل أنبأنا شعبة حدثني سهيل بن أبي صالح عن أبيه
كان رسول الله ﷺ إذا صلى ركعتي الفجر اضطجع
No references are available.
Takhrij, sahih.Umar bin Hisham narrated from Abu Hatim Razi, and he did not narrate except from someone he considered trustworthy, and the rest of the chain is sahih. The hadith has corroborating narrations, see the previous hadith.