Narrated: Umar
Messenger of Allah (ﷺ) used to fast three davs of each month:
1) Monday at the beginning of month,
2) The following Thursday,
3) Then the Thursday after that.

It was narrated from Hunaidah bin Khalid, from his wife, from one of the wives of the Prophet (ﷺ), that the Messenger of Allah (ﷺ) used to fast
1) Nine days of Zul Hijjah,
2) The day of Ashura
3) And three days of each month:
The first Monday of the month,
And (next) two Thursdays.

Narrated: Abu Qatada Ansari
Messenger of Allah (ﷺ) was asked about fasting on Monday, whereupon he (ﷺ) said:
It is (the day) when I was born and revelation was sent down to me.

PROPHET MUHAMMAD (ﷺ) said:
Deeds are presented on Monday and Thursday, and I love that my deeds be presented while I am fasting.

Narrated: Aisha
The Messenger of Allah (ﷺ) used to fast Shabaan and Ramdhaan, and he made sure to fast on Mondays and Thursdays.

Narrated: Abu Hurairah
Prophet (ﷺ) used to fast on Mondays and Thursdays.
It was said: “O Messenger of Allah (ﷺ), why do you (ﷺ) fast on Mondays and Thursdays?”
He (ﷺ) said: “On Mondays and Thursdays Allah forgives every Muslim except two who have forsaken one another.
He says: ‘Leave these two until they reconcile.’”

It was reported from the freed slave of Usamah bin Zaid, that he accompanied Usamah to the valley of Al-Qur'an, in pursuit of his wealth (camels).
He used to fast on Mondays and Thursdays.
He said to him: "Why do you fast on Mondays and Thursdays while you are an old man?"
He said: "The Prophet of Allah (ﷺ) used to fast on Mondays and Thursdays, and he (ﷺ) was asked about it.
He (ﷺ) said: 'The deeds of the servants (of Allah) are presented to Allah on Monday and Thursday."