Musnad Ahmad 30:3580

Ad-Dahhak narrated to us, from Al-Hasan ibn Yazid ibn Farrukh ad-Damri al-Madani, who said: I heard Abu Salamah saying: I bear witness that I heard Abu Hurairah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "No male slave or female slave shall swear a sinful oath at this pulpit, even if it were for a moist tooth-stick (miswak), but that the Fire will become mandatory for him."