Musannaf Ibn Abi Shayba 3:975

Abu al-Ahwas narrated to us, from Abu Ishaq, from al-Aizar ibn Hurayth, from Abu Nasr, who said: Ubayy ibn Ka'b said: The Messenger of Allah (ﷺ) led us in prayer. When he finished the prayer, he saw a scarcity of people in the mosque. He said, "Is so-and-so present?" We said, "Yes," until he counted three people. Then he said, "Indeed, there is no prayer more burdensome to the hypocrites than the 'Isha' prayer and the Fajr prayer. And if they knew what was in them, they would have come to them, even if crawling."