From Sahih Bukhari (Hadith 626):
Narrated `Aisha:Allah's Messenger (ﷺ) used to pray two light rak`at before the morning (compulsory) prayer after the day dawned and the Mu'adh-dhin had finished his Adhan. He then would lie on his right side till the Mu'adh-dhin came to pronounce the Iqama
Reference: Bukhari 626