The Messenger of Allah ﷺ said to him, "Fast one day, and you will have the reward of all the rest."He replied, "I am able to do more than that."He said, "Fast two days, and you will have the reward of all the rest."He replied, "I am able to do more than that."He said, "Fast three days, and you will have the reward of all the rest."He replied, "I am able to do more than that."He said, "Fast four days, and you will have the reward of all the rest."He replied, "I am able to do more than that."He said, "Then fast the best of fasts in the sight of Allah, the fast of Dawud . He used to fast one day and break his fast the next."
إن رسول الله ﷺ قال له: صم يوما ولك أجر ما بقي قال: أني أطيق أكثر من ذلك. قال: صم يومين، ولك أجر ما بقي قال: إني أطيق أكثر من ذلك. قال: صم ثلاثة أيام ولك أجر ما بقي قال: إني أطيق أكثر من ذلك. قال: صم أربعة أيام، ولك أجر ما بقي قال: إني أطيق أكثر من ذلك. قال: صم أفضل الصيام عند الله صوم داود عليه السلام، كان يصوم يوما ويفطر يوما.
Muslim narrated it in "Fasting" (1159:192) through multiple routes from Muhammad bin Jafar, from Shubah, from Ziyad bin Fayyad, who said, I heard Abu Iyad narrate from Abdullah bin Amr, who said (then he mentioned it).As for his saying, "Fast one day and you will have the reward of what remains," he means you will have the reward of the remaining days of the ten. Likewise his saying, "Fast two days and you will have the reward of what remains" means from the twenty. Likewise, "Fast three days and you will have the reward of what remains" means from the thirty. Likewise, "Fast four days and you will have the reward of what remains" means from the forty. This follows the principle of good deeds being multiplied tenfold. However, some said that fasting the fourth day carries no reward, because the month ends with the fasting of the three days. This is why Ibn Hibban omitted the mention of fasting the fourth day (3658), even though he narrated it from his teacher Ibn Khuzayma (2106), from Abdul-Warith bin Abdul-Samad, who said, my father narrated to us, Shubah narrated to us with his chain, while Ibn Khuzayma retained the mention of fasting the fourth day.Qurtubi said in "al-Mufhim" (3/224), "This is not the case, for if one traces its variations and joins them together, its form becomes coherent and its sequence proportionate, since the variation in it is not one of contradiction or conflicting reports. Rather, its variation comes down to some narrators mentioning what others left unmentioned, and detailing what others summarized." End of quote.