It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) used to fast three days every month: on the first Thursday of the month, on Monday of the following week, and again on Monday of the week after that.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5643
Hadith Gradingحکم دارالسلام:إسناده ضعيف، شريك بن عبد الله سيء الحفظ .