It was narrated that 'A’ishah (may Allah be pleased with her) said:
“كان النبي ﷺ يتحرَّى صوم الاثنين والخميس”
“The Prophet ﷺ was keen to fast on Mondays and Thursdays.”
[Reported by al-Tirmidhi, 745; al-Nasa’i, 2361; Ibn Majah, 1739; classed as authentic by al-Albani in Sahih al-Targhib, 1044]
It was also narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah ﷺ said:
“تُعرض الأعمال يوم الاثنين والخميس ، فأحب أن يعرض عملي وأنا صائم”
“Deeds are shown (to Allah) on Mondays and Thursdays, and I like my deeds to be shown when I am fasting.”
[Reported by al-Tirmidhi, 747; classed as authentic by al-Albani in Sahih al-Targhib, 1041]
Fasting monday & thursday is sunnah. May Allah give us strength and may we follow each and every sunnah of Rasool Allahi sallal alehe wasallam ameen