The Messenger of Allah ﷺ said, "The best fasting is the fasting of Dawud . He would fast one day and break his fast the next."
قال وفيما قرأ علينا أحمد بن منيع قال حدثنا هشيم قال أنبأنا حصين ومغيرة عن مجاهد
قال رسول الله ﷺ أفضل الصيام صيام داود عليه السلام كان يصوم يوما ويفطر يوما
Bukhari narrated it, in the Book of Fasting, chapter on fasting one day and breaking the fast the next, hadith no. 1978, from the hadith of Mughira as transmitted by him. It is also found in Kubra, hadith no. 2696.