Kurayb narrated: Ibn Abbas (may Allah be pleased with them both) and some people sent me to Sayyidah Umm Salamah (may Allah be pleased with her) to ask her on which days the Messenger of Allah (peace and blessings be upon him) fasted most. She said: "On Saturday and Sunday he (peace and blessings be upon him) fasted more than on any other days." And he (peace and blessings be upon him) used to say: "These two are days of festival for the polytheists, so I wish to differ from them."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20266