Musnad Ahmad 1007:126

Muhammad ibn al-Nawshajan—who is Abu Ja'far al-Suwaydi—narrated to us, al-Walid ibn Muslim narrated to us, from al-Awza'i, from Yahya ibn Abi Kathir, from 'Abdullah ibn Abi Qatadah, from his father, 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 complete its bowing nor its prostration," or he said: "He does not straighten his back in the bowing and the prostration."