Musnad Ahmad 30:3753

Abdullah ibn al-Harith narrated to us, Dawud ibn Qays narrated to us, from Musa ibn Yasar, from Abu Hurairah, that the Prophet, may Allah bless him and grant him peace, said: "The change in the breath of the mouth of the fasting person is more pleasant to Allah on the Day of Resurrection than the scent of musk."