Musannaf Ibn Abi Shayba 3:4988

Waki’ narrated to us, he said: Hisham bin ‘Urwah narrated to us, from his father, from Ibn ‘Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) 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."