Musannaf Ibn Abi Shayba 3:594

Ibn Mahdi narrated to us, from Qurrah, from al-Hasan, who said: Sa’id ibn al-Musayyib saw a man praying, and he was not completing his bowing (ruku’) nor his prostration (sujud). So he threw pebbles at him and said: "You have ruined your prayer."