Musnad Abu Ya'la al-Mawsili 6:14

Musa ibn Muhammad ibn Hayyan narrated to us, Muhammad ibn Abi al-Wazir Abu al-Mutarrif narrated to us, from Abdullah ibn Ja'far, from Ismail ibn Muhammad, from Amir ibn Sa'd, from his father, who said: The servant was commanded to prostrate upon seven of his limbs: his face, his palms, his knees, and his feet. Whichever of them he does not place [on the ground], he has fallen short.