´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."'
Hadith Referenceسنن نسائي / كتاب الصيام / 2360
Hadith Gradingالألبانی:حسن صحيح | زبیر علی زئی:حسن
Narrated Usamah ibn Zayd: The client of Usamah ibn Zayd said that he went along with Usamah to Wadi al-Qura in pursuit of his camels. He would fast on Monday and Thursday. His client said to him: Why do you fast on Monday and Thursday, while you are an old man? He said: The Prophet of Allah ﷺ used to fast on Monday and Thursday. When he was asked about it, he said: The works of the servants (of Allah) are presented (to Allah) on Monday and Thursday.
Shaykh Umar Farooq Saeedi
Benefits and Issues: In addition to this, in a hadith from Sahih Muslim and others, it is stated that the deeds of the night are raised to Him before the coming of the day, and the deeds of the day are raised to Him before the coming of the night. (Or, they are presented before Him.) (Sahih Muslim, Kitab al-Iman, Hadith: 179) In summary, these ahadith describe the system of the raising of deeds, which, without any delay or interruption, reach Allah ‘azza wa jall, and there can be a difference in the nature of these presentations: one is daily, and the other is weekly, which occurs on Mondays and Thursdays. Similarly, regarding Sha‘ban, it is also mentioned, so that presentation could be annual. And Allah knows best.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 2436