It is narrated from Hazrat Usama (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) used to fast on Mondays and Thursdays.
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 21791
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد حسن