Musnad Ahmad 1034:159

Waki' narrated to us, from Sufyan, from 'Asim, from Zirr, who said: I said to Hudhayfah: "At what time did you take the pre-dawn meal (suhur) with the Messenger of Allah, may Allah bless him and grant him peace?" He said: "It was daytime, except that the sun had not yet risen."