Hmam bin Yahya bin Dinar همام بن يحيى بن دينار · Fasting (Kitab Al-Siyam)

The Prophet (ﷺ) said: Do not fast on Saturday except what has been made obligatory on you; and if one of you can get nothing but a grape skin or a piece of wood from a tree, he should chew it.

The Messenger of Allah (ﷺ) said to me: The fast most liked by Allah is the one observed by Dawud (David), and the prayer dearer to Allah is the one offered by Dawud (David): he would sleep half the night, and stand (in prayer) one-third of it, and sleep one-sixth of it. He would go without fasting one day, and fast the other day.