Musnad Ahmad 28:15

Sufyan narrated to us, I heard ‘Amr, ‘Amr ibn Aws informed me, he heard it from ‘Abdullah ibn ‘Amr ibn al-‘As, who said: The Messenger of Allah (ﷺ) said: "The most beloved fasting to Allah is the fasting of David, and the most beloved prayer to Allah is the prayer of David. He used to sleep half of it, stand for a third of it, and sleep for a sixth of it. And he used to fast one day and break his fast one day."