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" (ID)
English reference (ID): Book 39, Hadith 268 (ID)
Arabic reference (ID): Book 40, Hadith 284 (ID)