Musannaf Ibn Abi Shayba 7:324

'Abd al-Rahim narrated to us, from Zakariya ibn Abi Za'idah, who said: 'Amir was asked about a man who had sworn an oath upon himself to emancipate two slaves from the children of Ishmael if he entered the house of so-and-so, and then he entered it. He said: "He must emancipate them as an expiation." The man said: "I cannot find them." He said: "Then fast for four consecutive months, two months for each slave, perhaps it may expiate something."