'Abdullah bin 'Amr bin Al-'As (May Allah be pleased with them) reported:
The Messenger of Allah (ﷺ) said, "Observing Saum (fasting) on three days of every month is equivalent to a full month's fasting."[Al-Bukhari and Muslim]. (ES)
App reference (ES): Book 9, Hadith 270 (ES)
Arabic/English book reference (ES): Book 9, Hadith 1260 (ES)