The Messenger of Allah ﷺ used to fast on Saturdays and Sundays more than on other days, and he would say, "They are festival days for the polytheists, and I love to differ from them."
كان رسول الله ﷺ يصوم يوم السبت ويوم الأحد أكثر مما يصوم من الأيام ويقول: إنهما يوما عيد المشركين، فأنا أحب أن أخالفهم.
Narrated by Ahmad (26750), and Tabarani in al-Kabir (23/283). Ibn Khuzayma graded it sahih (2167), as did Ibn Hibban (3616, 3646), and Hakim (1/436), all of them through routes from Abdullah bin Mubarak, who said, Abdullah bin Muhammad bin Umar bin Ali informed me, who said, my father narrated to us, from Kurayb, freedman of Ibn Abbas, from Umm Salama, and he mentioned it.Its isnad is hasan because of Abdullah bin Muhammad bin Umar bin Ali, who is the son of Abu Talib, for he is truthful. Ibn Madini said of him, "middling," and Daraqutni deemed him reliable. As for his father, Muhammad bin Umar bin Ali bin Abu Talib, a group narrated from him, and no one spoke against him. Ibn Hibban deemed him reliable, and Dhahabi said in al-Mizan, "I know of nothing wrong with him, nor have I seen any criticism of him."His statement, "so I like to differ from them," means in fasting, for they do not fast on their festivals, but rather indulge excessively in eating and drinking.