Musnad Ahmad 30:2405

Yahya ibn Sa'id narrated to us, from Ibn Abi Dhi'b, who said: 'Ajlan, the freed slave of al-Mushamma'il, narrated to me, saying: I heard Abu Hurayrah, from the Prophet, may God bless him and grant him peace, say: "Do not revile while you are fasting. If a person reviles you, then say: 'I am fasting.'"