Musnad Ahmad 27:1383

Waki’ narrated to us, Hisham bin ‘Urwah narrated to us, from his father, from Ibn ‘Umar, who said: The Messenger of Allah (ﷺ) said: "When the edge of the sun rises, delay the prayer until it fully appears. And when the edge of the sun sets, delay the prayer until it fully disappears."