"The Prophet used to try to fast on Mondays and Thursdays."
Jami` at-Tirmidhi Book 8, Hadith 64
← Previous Hadith the Messenger of Allah said: "Do not fast on Saturday except for what has been made obligatory upon you (by Allah). If one of you does not find but a grape peal or a tree's twig, then let him chew it."

Jami` at-Tirmidhi Book 8, Hadith 63 Next Hadith → "The Messenger of Allah would fast Saturdays, Sundays, and Mondays in one months, and Tuesdays, Wednesdays and Thursdays in the next month."

Jami` at-Tirmidhi Book 8, Hadith 65