Musnad Ahmad 403:30

Abdullah ibn Ahmad said: Dawud ibn Amr Abu Sulayman al-Dabbi narrated to us, he said: Abd al-Rahman ibn Abi al-Zinad narrated to us, from Abd al-Rahman ibn al-Harith, from Ubayd Allah ibn Abd Allah ibn Utbah, from Ibn Abbas, that al-Sa’b ibn Jaththama said: I said: "O Messenger of Allah, we raid the homes of the polytheists in the morning, and we strike the children beneath the bellies of the horses, and we do not perceive [it]?" He said: "They are of them."