Usamah bin Zaid said:
"I said: 'O Messenger of Allah, sometimes you fast, and you hardly ever break your hardly ever fast, except two days which, if you are fasting, you include them in your fast, and if you are not fasting, then you fast them on your own.' He said: 'Which two days?' I said: 'Monday and Thursday.' He said: 'Those are two days in which deeds are shown to the Lord of the worlds, and I like my deeds to be shown (to Him) when I am fasting."'