Musannaf Ibn Abi Shayba 7:38

'Abdah narrated to us, from Sa'id, from Qatadah, from Ibn 'Abbas, who said: "Whoever says 'I have a vow upon me' and does not specify it, then it is a binding oath; he must free a slave, or fast for two months, or feed sixty poor persons." He said: And Al-Hasan said: "It is an oath; he makes expiation for it."