Musannaf Ibn Abi Shayba 3:4963

Waki’ narrated to us, he said: Ibn Abi Dhi’b narrated to us, from al-Zuhri, from al-Sa’ib, he said: I saw ‘Umar ibn al-Khattab striking al-Munkadir for the two prostrations after the afternoon prayer, meaning the two rak’ahs.