What is the fast of Dawud?

Fasting every other day is called the "fast of Dawud." This name is given because Prophet Dawud (a.s.) used to fast in this manner. The Prophet (s.a.s.) himself gave this name to the fast and described its virtue as follows: "The most virtuous fast is the fast of Dawud (a.s.); he would fast one day and not fast the next." (Nesâî, Siyam, 76 [2388]; see also Bukhari, Ahadith al-Anbiya, 38 [3420], Sawm, 56 [1976]; Muslim, Siyam, 181 [1159]) The Prophet (s.a.s.) also said, "The fast most beloved to Allah is the fast of Prophet Dawud (a.s.)." (Bukhari, Tahajjud, 7 [1131]; Muslim, Siyam, 189-190 [1159])

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu