Musannaf Ibn Abi Shayba 7:354

Hafs narrated to us, from Hajjaj, from ‘Ata’ and from Mujahid, from Ibrahim, who said: If a man makes a vow upon himself to fast for a month, and he does not specify a month from the months, he said: If he wishes, he may fast them consecutively, and if he wishes, he may fast them separately.