Mishkat al-Masabih 7 Fasting كتاب الصوم 111

قَالَتْ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَصُومُ يَوْمَ السَّبْتِ وَيَوْمَ الْأَحَدِ أَكْثَرَ مَا يَصُومُ مِنَ الْأَيَّامِ وَيَقُولُ: «إِنَّهُمَا يَوْمَا عِيدٍ لِلْمُشْرِكِينَ فَأَنَا أُحِبُّ أَن أخالفهم» . رَوَاهُ أَحْمد

Umm Salama said that God’s messenger used to fast on Saturdays and Sundays more than on other days, and he used to say, “They are days of festival for the polytheists, and I like to act contrary to them.” Ahmad transmitted it.