Volume 2 · Book 26 · Hadith 590
Narrated Ibn 'Umar: I saw that Allah's Apostle used to ride on his Mount at Dhul Hulaifa and used to start saying, "Labbaik" when the Mount stood upright.
— Sahih Bukhari, Volume 2, Book 26: Pilgrimmage (Hajj), Hadith #590