Sayyiduna Abu Qatadah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "The worst thief among people is the one who steals from his prayer." The Companions said: O Messenger of Allah, how does he steal from his prayer? He (peace and blessings be upon him) said: "He does not complete its bowing (ruku') and its prostration (sujud)."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3270