Abu Dharr reported,
The Messenger of Allah ﷺ said, "Whoever fasts three days of each month, that is equivalent to fasting the entire year." Then Allah 
revealed confirmation of this in His Book, "Whoever comes with a good deed will have ten times the like thereof" [Anam 6:160]. So one day counts as ten days.
Ibn Majah 1708
Daifحدثنا سهل بن أبي سهل حدثنا أبو معاوية عن عاصم الأحول عن أبي عثمان
عن أبي ذر قال
قال رسول الله ﷺ من صام ثلاثة أيام من كل شهر فذلك صوم الدهر فأنزل الله عز وجل تصديق ذلك في كتابه {من جاء بالحسنة فله عشر أمثالها} فاليوم بعشرة أيام
Its isnad is daif. Tirmidhi transmitted it in the Fasting section, chapter on what has been said about fasting three days each month, no. 762, from the hadith of Abu Muawiya with this wording, and he said, hasan sahih. Abu Muawiya was followed by Abdul-Rahim bin Sulayman and others.Nasai transmitted it (4/219, no. 2412) with a sahih isnad from Asim, from Abu Uthman, from a man, from Abu Dharr with this wording. It has a sahih corroborating narration in Nasai and others from the hadith of Abu Hurairah with this wording, nos. 2409, 2408.