Musnad Ahmad 31:542

'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, that 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, and how does he steal from it?" He said: "He does not complete its bowing nor its prostration."