Musnad Ahmad 27:394

Muhammad ibn Ubayd narrated to us, Ubaydullah narrated to us, from Nafi, from Ibn Umar: "That the Messenger of Allah, peace and blessings of Allah be upon him, when he would put his foot into the stirrup, and his she-camel stood upright with him, he would begin the talbiyah from the mosque of Dhu al-Hulayfah."