Sahih al-Bukhari — Call to Prayers (Adhaan) — Hadith No: 809
Translation
Narrated Ibn `Abbas:The Prophet (ﷺ) 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