Musannaf Ibn Abi Shayba 7:293

Abdullah ibn Numayr narrated to us, he said: Zakariya ibn Abi Za'idah narrated to us, from al-Sha'bi, who said: Masruq was asked about this verse: "And whoever kills a believer by mistake, then the freeing of a believing slave and a compensation payment delivered to his family." Then he was asked regarding the fasting of two months: Is it for the slave alone, or for the compensation payment and the slave? He said: "Whoever does not find [the means], it is for the compensation payment and the slave."