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]. (ID)
App reference (ID): Book 9, Hadith 122 (ID)
Arabic/English book reference (ID): Book 9, Hadith 1112 (ID)