Musnad Ahmad 1153:2051

Rawh narrated to us, Ibn Jurayj narrated to us, he said: 'Ata' informed me, from Abu Salih al-Zayyat, that he heard Abu Hurayrah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When it is the day of fasting for one of you, let him not use obscene speech on that day, nor let him shout. If someone insults him or fights him, let him say: 'I am a fasting person, I am a fasting person.'"