Musnad Abu Ya'la al-Mawsili 154:53

Zuhayr narrated to us, Waki' narrated to us, Hisham ibn 'Urwah narrated to us, from his father, from Ibn 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the edge of the sun appears, then delay the prayer until it fully emerges. And when the edge of the sun disappears, then delay the prayer until it fully sets."