´Abu 'Uthman reported from a man, that Abu Dharrr said:` "I heard the Messenger of Allah say: "Whoever fasts three days of each month has fasted the month in full' or 'will have (the reward of) fasting the month."' 'Asim was in doubt.
Hadith ReferenceSunan an-Nasa'i / The Book of Fasting / Hadith: 2412