Bulugh al-Maram #801

Umm Salamah (RAA) narrated, The Messenger of Allah (ﷺ) used to fast more often on Saturdays and Sundays than on the other days. He would say, "They are the 'ids of the polytheists, and I love to act contrary to what they do." Related by An-Nasal and was rendered authentic by Ibn Khuzaimah, and the wording is his.