Musnad Ahmad 30:2970

Waki' narrated to us, al-A'mash narrated to us, from Abu Salih, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the heaviest of prayers upon the hypocrites are the Isha prayer and the Fajr prayer. And if they knew what was in them, they would have come to them, even if crawling."