بُلُوغُ الْمَرَامِ
Bulugh al-Maram 365
The Book of Prayer
وَعَنْ عَائِشَةَ -رَضِيَ اَللَّهُ عَنْهَا- قَالَتْ : { كَانَ اَلنَّبِيُّ - صلى الله عليه وسلم -إِذَا صَلَّى رَكْعَتَيْ اَلْفَجْرِ اِضْطَجَعَ عَلَى شِقِّهِ اَلْأَيْمَنِ } رَوَاهُ اَلْبُخَارِيُّ . [1]
Narrated 'Aishah (RA): The Prophet (ﷺ) used to lie down on his right side, after praying the two (voluntary) Rak'at of the Fajr (prayer). [Reported by al-Bukhari].
Sunnah.com reference: Book 2, Hadith 269