The Messenger of Allah ﷺ said, "The best fasting is the fasting of my brother Dawud. He used to fast one day and break his fast the next, and he did not flee when he met the enemy."He said, the Messenger of Allah ﷺ said, "He has not fasted who fasts perpetually."
حدثنا وكيع حدثنا سفيان ومسعر عن حبيب بن أبي ثابت عن أبي العباس المكي
قال رسول الله ﷺ أفضل الصوم صوم أخي داود كان يصوم يوما ويفطر يوما ولا يفر إذا لاقى قال قال رسول الله ﷺ لا صام من صام الأبد
Its isnad is sahih. Sufyan is Thawri. The hadith is an abridgment of 6766. As for its statement at the end, "And the Messenger of Allah ﷺ said," in H it reads "he said, he said" instead of "and he said," and we have adopted what is in K and M.