Musnad Ahmad 1007:55

Isma'il narrated to us, Muhammad ibn Ishaq narrated to us, Ma'bad ibn Ka'b ibn Malik narrated to me, from Abu Qatadah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Beware of excessive swearing in selling; for it promotes the sale, then it wipes out the blessing."