Sahih Muslim 1650
Sahih Muslim · Book 27: The Book of Oaths · It is recommended for the one who swears an oath then sees that something else is better than it; To do that which is better and offer expiation for his oath
This hadith is narrated on the authority of Suhail with the same chain of transmitters (with these words):
وَحَدَّثَنِي الْقَاسِمُ بْنُ زَكَرِيَّاءَ ، حَدَّثَنَا خَالِدُ بْنُ مَخْلَدٍ ، حَدَّثَنِي سُلَيْمَانُ، - يَعْنِي ابْنَ بِلاَلٍ - حَدَّثَنِي سُهَيْلٌ ، فِي هَذَا الإِسْنَادِ بِمَعْنَى حَدِيثِ مَالِكٍ " فَلْيُكَفِّرْ يَمِينَهُ وَلْيَفْعَلِ الَّذِي هُوَ خَيْرٌ " .
" He should expiate for (breaking) the vow and do that which is better."