Ibn 'Umar Radiyallahu 'Anhu reports.. "Hafsah (my sister, Ummul Mu-mineen) related to me that when the time of fajr began (subh saadiq), and the mu-adh-dhin called out the adhaan, Rasulullah Sallallahu 'Alayhi Wasallam performed two short rak-'ahs" (ES)
English reference (ES): Book 39, Hadith 268 (ES)
Arabic reference (ES): Book 40, Hadith 284 (ES)