Musnad Ahmad 21:17

Qutaybah ibn Sa'id narrated to us, Bakr ibn Mudar al-Qurashi narrated to us, from Ibn al-Had, from Muhammad ibn Ibrahim ibn al-Harith, from 'Amir ibn Sa'd, from al-'Abbas ibn 'Abd al-Muttalib, that he heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "When the servant prostrates, seven limbs prostrate with him: his face, his palms, his knees, and his feet."