عَنِ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُ قَالَ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ يَصُومُ ثَلَاثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ، الْخَمِيسَ مِنْ أَوَّلِ الشَّهْرِ وَالْإِثْنَيْنِ الَّذِي يَلِيهِ وَالْإِثْنَيْنِ الَّذِي يَلِيهِ
It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) used to fast on these three days every month: the first Thursday of the month, the following Monday, and then the Monday after that.