Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Fast one day, and you will have the reward of the rest." He said: "I can manage more than that." He said: "Fast two days, and you will have the reward of the rest." He said: "I can manage more than that." So he said: "Then fast three days, and you will have the reward of the rest." He said: "I can manage more than that." He said: "Then fast four days, and you will have the reward of the rest." He said: "I can manage more than that." So he said: "Then keep the fast that is best with Allah, the fasting of Dawud (peace be upon him): he would fast one day and not fast the next."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8451