Narrated Ibn `Abbas: The Prophet was ordered (by Allah) to prostrate on seven parts and not to tuck up the clothes or hair (while praying). Those parts are: the forehead (along with the tip of nose), both hands, both knees, and (toes of) both feet.
Hadith Referenceصحيح البخاري / كتاب الأذان (صفة الصلوة) / 809
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة
Narrated Al-Bara' bin `Azib: (He was not a liar) We used to pray behind the Prophet and when he said, "Sami`a l-lahu liman hamidah", none of us would bend his back (to go for prostration) till the Prophet had placed his, forehead on the ground.
Hadith Referenceصحيح البخاري / كتاب الأذان (صفة الصلوة) / 811
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة