Musnad Ahmad 935:9

Abdullah narrated to us, Khalaf ibn Hisham al-Bazzar and Abu Bakr ibn Abi Shaybah narrated to us, they said: Abu al-Ahwas narrated to us, from Abu Ishaq, from al-Izar ibn Hurayth, from Abu Basir, he said: My father said: The Messenger of Allah (peace and blessings of Allah be upon him) led us in the dawn prayer. When he finished the prayer, he saw a scarcity of people in the mosque, so he said: "Is so-and-so present?" We said: "Yes." He continued until he had counted three men. Then he said: "Indeed, there is no prayer heavier upon the hypocrites than the last night prayer (Isha) and the dawn prayer (Fajr)." And he mentioned the rest of the hadith in its entirety.