It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the limbs on which the Messenger of Allah (peace and blessings be upon him) was commanded to prostrate are seven: "the two hands, the two knees, the edges of the toes and the forehead," and he was forbidden "to tuck up the clothes and the hair."
Sufyan says that Ibn Tawus added this: he placed his hand on his forehead, then passed it down over his nose until he brought it to the tip of his nose, and said: My father used to count this (whole) as one bone.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2648