It wasnarrated from Rabi'ah bin Ghaz that he asked 'Aishah about thefasting of the Messenger of Allah (ﷺ). She said:
"He used to makesure he fasted on Mondays and Thursdays." (PT)
English reference (PT): Vol. 1, Book 7, Hadith 1739 (PT)
Arabic reference (PT): Book 7, Hadith 1811 (PT)