Musnad Ahmad 32:1855

Yahya ibn Ghaylan narrated to us, Al-Mufaddal ibn Fadalah narrated to us, he said: Uqayl narrated to me, from Ibn Shihab, that he narrated to him, from Anas ibn Malik, who said: "When the Messenger of Allah, peace and blessings of Allah be upon him, would depart before the sun had declined, he would delay the Dhuhr prayer until the time of the Asr prayer. Then he would descend and combine them. And when the sun had declined before he departed, he would pray Dhuhr, then he would mount."