Musnad Ahmad 30:939

Bahz narrated to us, Hammam narrated to us, Qatadah narrated to us, from Bashir ibn Nahik—and I do not think it is except from an-Nadr ibn Anas, from Bashir ibn Nahik—from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The change in the breath of the fasting person is more pleasant to Allah on the Day of Resurrection than the scent of musk."