Musnad Ahmad 456:8

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj informed us, he said: I heard Abu Sa'id al-A'ma informing about a man who is called: al-Sa'ib, the freed slave of the Persians—and Ibn Bakr said: the freed slave of Persia, and Hajjaj said: the freed slave of the Persian—from Zayd ibn Khalid, that 'Umar ibn al-Khattab saw him while he was Caliph performing two rak'ahs after the 'Asr prayer. So he walked toward him and struck him with the whip while he was still praying. When he finished, Zayd said: "O Commander of the Faithful, by Allah, I will never abandon them after I saw the Messenger of Allah, peace and blessings of Allah be upon him, performing them." He said: So 'Umar sat down with him and said: "O Zayd ibn Khalid, were it not that I fear people would take them as a ladder to prayer until the night, I would not have struck [anyone] for them."