(70) باب مَنْ قَالَ الاِثْنَيْنِ وَالْخَمِيسِ
Narrated Hafsah, Ummul Mu'minin:
The apostle of Allah (ﷺ) used to fast three days every month: Monday, Thursday and Monday in the next week.
Reference: Sunan Abi Dawud 2451
In-book reference: Book 14, Hadith 139
English translation: Book 13, Hadith 2445