It is narrated from Hazrat Abu Dharr (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Whoever fasts three days every month, it is as if he has fasted for a lifetime.
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 21301
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد ضعيف ، أبو عثمان لم يسمعه من أبى ذر