Musnad Ahmad 935:1

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, I heard Abu Ishaq, that he heard ‘Abdullah ibn Abi Basir narrating from Ubayy ibn Ka‘b, that he said: The Messenger of Allah (peace and blessings of Allah be upon him) prayed the morning prayer, then he said: "Is so-and-so present?" They said: "No." He said: "Is so-and-so present?" They said: "No." He said: "Is so-and-so present?" They said: "No." He said: "Indeed, these two prayers are among the heaviest of prayers for the hypocrites. If they knew what was in them, they would have come to them even if by crawling. The first row is like the row of the angels; if you knew its virtue, you would have raced to it. A man’s prayer with another man is more pure than his prayer alone, and his prayer with two men is more pure than his prayer with one man, and whatever is more, it is more beloved to Allah."