Musannaf Ibn Abi Shayba 3:590

Hushaym narrated to us, he said: Yunus informed us, from al-Hasan, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "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 his prayer?" He said: "He does not perfect its bowing nor its prostration."