Musnad Ahmad 117:3

'Abd al-Rahman ibn Mahdi narrated to us, Zuhayr—meaning Ibn Muhammad—narrated to us, from Muhammad ibn 'Amr ibn Halhalah, from Nu'aym ibn 'Abd Allah, from Ibn Takhfah al-Ghifari, who said: My father informed me, saying: The Messenger of Allah (may Allah bless him and grant him peace) had guests along with a group of people. He said: So we spent the night with him. The Messenger of Allah (may Allah bless him and grant him peace) went out during the night to check on them, and he saw him lying on his stomach. He nudged him with his foot and woke him up, then said: "This is the lying position of the people of the Fire."