Abdullah bin Amr bin As reported,
The Messenger of Allah ﷺ said to me, "Fast one day and you will have the reward of what remains."He kept counting until he reached four or five days. Shuba was uncertain about the number.He said, "Fast the best of fasts, the fast of Dawud 
. He used to fast one day and break his fast the next."
Ahmad 6915
Sahihحدثنا روح حدثنا شعبة عن زياد بن فياض عن أبي عياض سمعت
عبد الله بن عمرو يقول
قال لي رسول الله ﷺ صم يوما ولك أجر ما بقي حتى عد أربعة أيام أو خمسة شعبة يشك قال صم أفضل الصوم صوم داود عليه السلام كان يصوم يوما ويفطر يوما
Its chain of transmission is sahih. Ziyad bin Fayyad, with a fatha on the fa and a doubled ya, ending in a dotted dal, Khuzai, Kufi, is trustworthy. He was deemed trustworthy by Ibn Madini, Ibn Main, Abu Hatim, and others. Bukhari gave his biography in al-Kabir, 1/2/334, and Ibn Hibban mentioned him in al-Thiqat, p. 467.Abu Ayyad, with a kasra on the plain ain and a light ya, is Amr bin Aswad Ansi, as we established as more likely in 6497, and we add here that Ibn Hibban mentioned him in al-Thiqat, p. 279.This hadith was narrated by Muslim, 1/321, through Muhammad bin Jafar, and by Nasai, 1/325, through Ibn Jafar as well, and 327, through Hajjaj bin Muhammad, both of them from Shubah, with this same chain. Tayalisi also narrated it, 2288, from Shubah as well, but he presented it in abbreviated form. This hadith, like the one before it, is one of the narrations of the story of Abdullah bin Amr, 6477, and we neglected to point that out there as well.