Musnad Ahmad Hadith 7087

It is narrated from Ibn Amr, may Allah be pleased with him, that once I presented myself before the Noble Prophet, peace and blessings be upon him, and said, "O Messenger of Allah! Give me a command regarding fasting." The Noble Prophet, peace and blessings be upon him, said: "If you fast one day, you will get the reward of nine." I requested an increase in this, so he said: "If you fast two days, you will get the reward of eight." I requested further increase, so he said: "If you fast three days, you will get the reward of seven." Then the Noble Prophet, peace and blessings be upon him, kept decreasing continuously until finally he said: "The best way of fasting is that of Dawood, peace be upon him, so fast one day and leave one day."