Sahih al Bukhari 7538
Oneness, Uniqueness of Allah (Tawheed)
حَدَّثَنَا آدَمُ، حَدَّثَنَا شُعْبَةُ، حَدَّثَنَا مُحَمَّدُ بْنُ زِيَادٍ، قَالَ سَمِعْتُ أَبَا هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم يَرْوِيهِ عَنْ رَبِّكُمْ، قَالَ " لِكُلِّ عَمَلٍ كَفَّارَةٌ، وَالصَّوْمُ لِي وَأَنَا أَجْزِي بِهِ، وَلَخَلُوفُ فَمِ الصَّائِمِ أَطْيَبُ عِنْدَ اللَّهِ مِنْ رِيحِ الْمِسْكِ ".
Narrated Abu Huraira:The Prophet (ﷺ) said that your Lord said, "Every (sinful) deed can be expiated; and the fast is for Me, so I will give the reward for it; and the smell which comes out of the mouth of a fasting person, is better in Allah's Sight than the smell of musk." (See Hadith No)
Text from the public-domain hadith-api dataset. Numbering follows that dataset and can differ by a few from some printed editions — check the wording, not only the number.