Musannaf Ibn Abi Shayba 3:4011

Waki’ narrated to us, he said: Shu’bah narrated to us, from Muhammad ibn al-Munkadir, from Sa’id ibn al-Musayyib, he said: Umar saw a man who lay down after the two rak’ahs, so he said: "Pelt him with pebbles," or "Will you not pelt him with pebbles?"