It is narrated from Sayyiduna Abbas bin Abd al-Muttalib (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When a person prostrates, seven limbs prostrate along with him: his face (forehead), his palms, his knees and his two feet."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2641