Volume 4 · Book 52 · Hadith 117
Narrated Ibn'Umar: When the Prophet put his feet in the stirrup and the she-camel got up carrying him he would start reciting Talbiya at the mosque of Dhul-Hulaifa.
— Sahih Bukhari, Volume 4, Book 52: Fighting for the Cause of Allah (Jihaad), Hadith #117