Musnad Ahmad 457:27

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, who said: We were with 'Umar ibn 'Abd al-'Aziz, and he delayed the 'Asr prayer one time. 'Urwah ibn al-Zubayr said to him: Bashir ibn Abi Mas'ud al-Ansari narrated to me that al-Mughirah ibn Shu'bah delayed the prayer one time—meaning the 'Asr—so Abu Mas'ud said to him: "By Allah, O Mughirah, you know that Gabriel, peace be upon him, descended and prayed, and the Messenger of Allah, may Allah bless him and grant him peace, prayed, and the people prayed with him. Then he descended and prayed, and the Messenger of Allah, may Allah bless him and grant him peace, prayed, and the people prayed with him, until he counted five prayers." 'Umar said to him: "Look at what you are saying, O 'Urwah. Is it Gabriel who established the time of the prayer?" 'Urwah said: "That is how Bashir ibn Abi Mas'ud narrated to me." 'Umar continued to learn the time of the prayer by that sign until he departed from the world.