Musnad Ahmad 25:893

Aswad narrated to us, Ayyub ibn Utbah narrated to us, from Yahya ibn Abi Kathir, from Ata, from Ibn Abbas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade the sale of *gharar* (uncertainty/risk)." Ayyub said: And Yahya explained the sale of *gharar*, saying: "Indeed, among the *gharar* is the diver’s strike (a contract where the diver receives a lump sum for whatever he brings up in one dive), the sale of a runaway slave, the sale of a stray camel, the sale of *gharar* is what is in the bellies of livestock, the sale of *gharar* is the soil of mines, and the sale of *gharar* is what is in the udders of livestock, unless it is by measurement."