Musannaf Ibn Abi Shayba 3:5013

Waki’ narrated to us, he said: Hanzala al-Jumahi narrated to us, from Tawus, that a man made a vow to prostrate upon the forehead of the Prophet, may God bless him and grant him peace. So he fulfilled it. The Prophet, may God bless him and grant him peace, sat facing the Qibla, then the man stood behind him and said: "Like this," with his forehead, and he prostrated upon it.