Musnad Abu Ya'la al-Mawsili 160:229

Abu Sa’id al-Qawariri narrated to us, Yahya narrated to us, from ‘Ubayd Allah ibn ‘Umar, Sa’id al-Maqburi narrated to us, from ‘Umar ibn Abi Bakr ibn ‘Abd al-Rahman, from his father, that ‘Ammar ibn Yasir prayed two rak’ahs and made them light. ‘Abd al-Rahman ibn al-Harith said to him, "O Abu al-Yaqzan, I see that you have made them light." He said, "I hastened through them to avoid the whisperings (waswas). And I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'Indeed, a man may perform the prayer, and perhaps there is nothing for him from it except a tenth of it, or a ninth of it, or an eighth of it, or a seventh of it, or a sixth of it,'" until he reached the full number.