Musnad Ahmad 935:2

Waki’ narrated to us, from Sufyan, from Abu Ishaq, from ‘Abdullah ibn Abi Basir, from Ubayy ibn Ka’b, who said: The Messenger of Allah (peace and blessings of Allah be upon him) prayed the dawn prayer. When he finished the prayer, he said: "Is so-and-so present?" The people remained silent. They said: "Yes," but he had not attended. The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the most burdensome prayers for the hypocrites are the night prayer (Isha) and the dawn prayer (Fajr). If they knew what was in them, they would have come to them even if by crawling. And the first row is like the row of the angels; if you knew its virtue, you would have raced to it. Indeed, your prayer with two men is purer than your prayer with one man, and your prayer with one man is purer than your prayer alone. And whatever is more in number is more beloved to Allah." Waki’ said: ‘Abdullah ibn Abi Basir is [of the tribe of] Ghanm.