On the authority of Usamah Ibn Zaid (رضي الله عنه) who said, I asked; Oh Messenger of Allah, verily you fast until it seems that you don’t break it, and you break the fast until it seems that you don’t fast except for two days, if they come in your fast and other than that you fast both of them.
He said: “And which two days are these?”
I said: On Monday and Thursday.
He then said: “In these two days the actions are displayed to the Lord of the worlds. Therefore I would like that my actions are displayed and I am fasting.”
Narrated by Imam Nasaee. Imaam Muqbil Ibn Haadi declared it to be Hasan.