بُلُوغُ الْمَرَامِ
Bulugh al-Maram 366
The Book of Prayer
وَعَنْ أَبِي هُرَيْرَةَ - رضى الله عنه - قَالَ : قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ إِذَا صَلَّى أَحَدُكُمْ اَلرَّكْعَتَيْنِ قَبْلَ صَلَاةِ اَلصُّبْحِ , فَلْيَضْطَجِعْ عَلَى جَنْبِهِ اَلْأَيْمَنِ } رَوَاهُ أَحْمَدُ , وَأَبُو دَاوُدَ , وَاَلتِّرْمِذِيُّ وَصَحَّحَهُ . [1]
Narrated Abu Hurairah (RA): Allah's Messenger (ﷺ) said: "If any of you prays the two Rak'at before the dawn prayer, he should lie on his right side." [Reported by Ahmad, Abu Dawud and at-Tirmidhi, the latter graded it Sahih (authentic)].
Sunnah.com reference: Book 2, Hadith 270