Mukhtasar Muslim (Abridged) 629 — Best fast is that of Dawud: fasting

Narrated by Sayyiduna Abdullah bin Amr (may Allah be pleased with them both): The Messenger of Allah (peace and blessings be upon him) said: The most beloved fast to Allah is the fast of Sayyiduna Dawud (peace be upon him), and the most beloved prayer is the prayer of Dawud (peace be upon him). (Sayyiduna Dawud [peace be upon him]) used to sleep half the night, stand (in prayer) for one third of it (i.e., perform Tahajjud), and then sleep for one sixth of it. And he used to fast one day and break his fast the next day.