Musnad Abu Ya'la al-Mawsili 105:14

Al-Qawariri narrated to us, Yahya bin Sa'id narrated to us, from 'Ubaydullah bin 'Umar, Sa'id al-Maqburi narrated to me, from 'Umar bin Abi Bakr bin 'Abd al-Rahman bin al-Harith, from his father, that 'Ammar bin Yasir prayed two rak'ahs. 'Abd al-Rahman bin 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). I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'Indeed, a man may perform a 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, or a fifth of it,'" until he reached the full number.