Musnad Abu Ya'la al-Mawsili 42:338

Zuhayr narrated to us, Affan narrated to us, Hammad narrated to us, Ali bin Zayd informed us, from Sa'id bin al-Musayyib, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, the worst of people in theft is the one who steals from his prayer." They said: "O Messenger of Allah, how does he steal from it?" He said: "He does not complete its bowing (ruku') nor its prostration (sujud)."