ENOfficial
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.
Source: sunnah.com
Other translations
Arabic
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
sunnah.com