Musannaf Ibn Abi Shayba 3:320

Abu Bakr narrated to us, he said: Isma'il bin 'Ayyash narrated to us, from 'Abd al-'Aziz bin 'Ubayd Allah, he said: I said to Wahb bin Kaysan, "O Abu Nu'aym, why do you not place your forehead and your nose firmly upon the ground?" He said, "That is because I heard Jabir bin 'Abd Allah saying: 'I saw the Messenger of Allah, may Allah bless him and grant him peace, prostrating on the upper part of his forehead, at the hairline.'"