Hadith 10, 40 Hadith Qudsi
On the authority of Abu Hurayrah (may Allah be pleased with him) from the Prophet (ﷺ), who said:
Allah (mighty and sublime be He) says:
Fasting is Mine and it is I who give reward for it. [A man] gives up his sexual passion, his food and his drink for my sake. Fasting is like a shield, and he who fasts has two joys: a joy when he breaks his fast and a joy when he meets his Lord. The change in the breath of the mouth of him who fasts is better in Allah's estimation than the smell of musk.

It was related by al-Bukhari (also by Muslim, Malik, at-Tirmidhi, an-Nasa'i and Ibn Majah).
عَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ، عَنْ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: 

" يَقُولُ اللَّهُ عَزَّ وَجَلَّ: الصَّوْمُ لِي، وَأَنَا أَجْزِي بِهِ، يَدَعُ شَهْوَتَهُ وَأَكْلَهُ وَشُرْبَهُ مِنْ أَجْلِي، وَالصَّوْمُ جُنَّةٌ(1)، وَلِلصَّائِمِ فَرْحَتَانِ: فَرْحَةٌ حِينَ يُفْطِرُ، وَفَرْحَةٌ حِينَ يَلْقَى رَبَّهُ، وَلَخُلُوفُ(2) فَمِ الصَّائِمِ أَطْيَبُ عِنْدَ اللَّهِ مِنْ رِيحِ الْمِسْكِ". 
رواه البخاري (وكذلك مسلم ومالك والترمذي النسائي وابن ماجه)