Shama-il Tirmidhi Chapter 042, Hadith Number 007 (287).

Aayeshah Radhiyallahu anha reports: "Rasulullah Sallallahu Alayhi wasallam (often) gave importance to the fasting on Mondays and Thursdays"

Commentary
In some narrations the reason for giving importance to these days are mentioned. One reason will be mentioned in a hadith of Sayidina Abu Hurayrah Radhiyallahu anhu which follows in this chapter, that deeds are presented on these days. In a narration in Sahih Muslim regarding Monday, it has been stated that "I was born on a Monday, the revealing of the Quran to me also began on a Monday". In one hadith it stated that "Allah Ta'alah forgives the sins of every Muslim (provided the rules apply) on Mondays and Thursdays". It is said regarding those two people who are on bad terms "hold the granting of their forgiveness till they reconcile among themselves".