Abu Dharr reported,
The Messenger of Allah ﷺ said, "Whoever fasts three days of every month, that is equivalent to fasting the entire year." Then Allah 
revealed confirmation of this in His Book, "Whoever brings a good deed shall have ten times its like" [Anam 6:160], one day counting as ten days.
Tirmidhi 762
Daifحدثنا هناد حدثنا أبو معاوية عن عاصم الأحول عن أبي عثمان النهدي
عن أبي ذر قال
قال رسول الله ﷺ من صام من كل شهر ثلاثة أيام فذلك صيام الدهر فأنزل الله عز وجل تصديق ذلك في كتابه {من جاء بالحسنة فله عشر أمثالها} اليوم بعشرة أيام
[Its chain of transmission is daif] Ibn Majah also recorded it in the Book of Fasting, chapter on what has been reported about fasting three days from every month, hadith no. 1708, through the hadith of Abu Muawiya al-Darir with this wording, and Abu Uthman heard it from an unknown man, from Abu Dharr, with this wording.