It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) was commanded to prostrate on seven bones and was forbidden to tuck up his hair and his clothes. The seven bones on which he was commanded to prostrate are: the two hands, the two knees, the two feet and the forehead.
This is the wording of Abu al-Rabi's hadith; and in the hadith of Ibn Abbas (may Allah be pleased with them both) it is that the Prophet (peace and blessings be upon him) was commanded to prostrate on seven (bones/limbs) and was forbidden to tuck up clothes and hair (the words are either thawban (a garment) or thiyabahu (his clothes)).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2640