Abu Hurairah (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) said, "When anyone of you has performed two of Sunnah before the Fajr prayer, he should lie down on his right side."[Abu Dawud and At-Tirmidhi]. (ES)
App reference (ES): Book 9, Hadith 122 (ES)
Arabic/English book reference (ES): Book 9, Hadith 1112 (ES)