Kurayb relates that some people sent me to Sayyidah Umm Salamah (may Allah be pleased with her) to ask her: On which days did the Messenger of Allah (peace and blessings be upon him) fast the most? She said: On Saturday and Sunday, and he (peace and blessings be upon him) would say: "These two are festival days of the polytheists, so I like to differ from them."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19920