Musnad Ahmad Hadith 21272

It is narrated from Ubayy bin Ka'b (may Allah be pleased with him) that once the Holy Prophet (ﷺ) said: If people knew the reward of offering the Isha and Fajr prayers in congregation, they would surely participate in them even if they had to come crawling. It is narrated from Ubayy bin Ka'b (may Allah be pleased with him) that once the Holy Prophet (ﷺ) led the Fajr prayer, and after the prayer, he asked, mentioning some people by name one by one, "Is such and such a person present?" Each time, the people replied, "No." The Holy Prophet (ﷺ) said: These two prayers (Isha and Fajr) are the heaviest on the hypocrites. If they knew their reward, they would surely participate in them even if they had to come crawling.... Then the narrator mentioned the entire hadith.