Musannaf Ibn Abi Shayba 1:30

Abu al-Ahwas narrated to us, from Adam ibn Ali, who said: I heard Ibn Umar saying: "Indeed, there are people who will be called 'the deficient ones' on the Day of Resurrection." A man said: "Who are they, O Abu Abd al-Rahman?" He said: "One of them would diminish his purification and his turning (looking around) in his prayer."