Musannaf Ibn Abi Shayba 7:32

Abu Bakr narrated to us, he said: Sufyan ibn ‘Uyaynah narrated to us, from ‘Ata’, from Sa‘id ibn Jubayr, from Ibn ‘Abbas, he said: Regarding a vow (nadhr) that is not specified: it is the most severe of oaths, and upon him is the most severe of expiations.