Musannaf Ibn Abi Shayba 7:15

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Mansur, from Sa’id ibn Jubayr, from Ibn Abbas (may Allah be pleased with them both) regarding a man who swears by a vow or by prohibition (haram). He said: "He has not spared effort in making it severe upon himself; he shall free a slave, or fast for two months, or feed sixty poor persons." He said: So I asked Ibrahim and Mujahid, and they both said: "If he does not find (the means), he shall feed ten poor persons."