Sâlim b. ‘Ubayd reports, “I used to stay in the house of Abû Bakr. One night, he prayed for as long as Allâh willed him to. He then said [to me], ‘Go and see if al-fajr has started,’ so I went, returned and said, ‘Whiteness has risen in the sky.’ So he prayed for as long as Allâh willed, then said [again], ‘Go and see if al-fajr has started.’ I went out, returned and said, ‘[The light] is spreading out and becoming reddish,’ to which he said, ‘Now bring me my drink (i.e. my pre-dawn meal, al-suhûr).’”
Al-Dâraqutnî, Al-Sunan 2:166. Al-Dâraqutnî grades its chain of transmission sahîh.