Musannaf Ibn Abi Shayba 3:4960

Waki’ narrated to us, from Shu’bah, from Sa’d ibn Ibrahim, who said: I heard ‘Ubayd Allah ibn Rafi’ ibn Khadij narrating from his father, who said: ‘Umar ibn al-Khattab saw me one day while I was praying after the ‘Asr prayer. He waited for me until I finished, then he said: "What is this prayer?" I said: "I missed a part of the prayer [earlier]." ‘Umar said: "Had I known that you were praying after ‘Asr, I would have done such and such."