Sahih Bukhari Volume 1, Book 11, Hadith Number 599

Narated By ‘Aisha : Allah’s Apostle used to pray two light Rakat before the morning (compulsory) prayer after the day dawned and the Mu’adhdhin had finished his Adhan. He then would lie on his right side till the Mu’adhdhin came to pronounce the Iqama.