Hunaydah al-Khuza'i reported on the authority of her mother who said: I entered upon Umm Salamah and asked her about fasting. She said: The Messenger of Allah ﷺ used to command me to fast three days every month beginning with Monday or Thursday. (Using translation from Abū Dāʾūd 2452)
كَانَ النَّبِيُّ ﷺ يَأْمُرُنِي أَنْ أَصُومَ ثَلَاثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ أَوَّلُهَا الِاثْنَيْنِوَالْجُمُعَةُ وَالْخَمِيسُ