Musannaf Ibn Abi Shayba 3:922

Ibn Fudayl narrated to us, from Layth, from ‘Abdah, from ‘Abd al-Rahman ibn Ghanam, who said: I wrote to ‘Umar asking him about the time of the ‘Asr prayer. He wrote back to me: Pray ‘Asr when the sun is between the two twilights.