Musnad Ahmad 21:2

'Abd al-Rahman ibn Mahdi narrated to us, 'Abd Allah ibn Ja'far narrated to us, from Isma'il ibn Muhammad, from 'Amir ibn Sa'd, from al-'Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When a man prostrates, seven limbs prostrate with him: his face, his two palms, his two knees, and his two feet."