Asma bint Wathilah (may Allah have mercy on her) narrates from her father that he used to fast on Monday and Thursday, and he would say: the Messenger of Allah (peace and blessings be upon him) also used to fast these two days and would say: "On these two days deeds are presented before Allah, the Blessed and Exalted."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18453