Musannaf Ibn Abi Shayba 3:583

Affan narrated to us, he said: Hammad bin Salamah narrated to us, he said: Ali bin Zaid informed us, from Sa'id bin al-Musayyib, from Abu Sa'id al-Khudri, that the Prophet (ﷺ) 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 nor its prostration."