I heard the Messenger of Allah ﷺ say, "The month of patience and three days of every month is equivalent to fasting the entire year."
أخبرنا زكريا بن يحيى قال حدثنا عبد الأعلى قال حدثنا حماد بن سلمة عن ثابت عن أبي عثمان أن
سمعت رسول الله ﷺ يقول شهر الصبر وثلاثة أيام من كل شهر صوم الدهر
Its chain of transmission is sahih. Ahmad narrated it, 2/263, 284, 513, from the hadith of Hammad bin Salama with the same wording, al-Kubra, no. 2716. Bukhari narrated it, no. 1178, and Muslim, no. 85/721, from the hadith of Abu Uthman Nahdi, from Abu Hurairah, who said, "My close companion charged me to observe three things, to fast three days from every month, to pray the two units of Duha prayer, and to pray witr before I sleep."