Musnad Ahmad 21:7

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah informed us, from Yazid ibn Abdullah ibn al-Had, from Muhammad ibn Ibrahim al-Taymi, from Amir ibn Sa'd, from al-Abbas ibn Abd al-Muttalib, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the son of Adam prostrates, seven limbs prostrate with him: his face, his two palms, his two knees, and his two feet."