Musnad Ahmad 27:247

Yahya—meaning Ibn Sa'id—narrated to us, Hisham ibn 'Urwah narrated to us, my father informed me, Ibn 'Umar informed me, from the Prophet, may Allah bless him and grant him peace, who said: "When the edge of the sun rises, then delay the prayer until it fully appears. And when the edge of the sun sets, then delay the prayer until it fully sets."