It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) "was commanded to prostrate on seven bones, that is, seven limbs, and that one should not tuck up his clothes or hair during the prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2677