Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever makes a vow without naming it, its expiation is that of an oath; whoever makes a vow of disobedience, its expiation is also that of an oath; whoever makes a vow that he is unable to fulfil, its expiation is also that of an oath; and whoever makes a vow that he is able to fulfil, let him fulfil it."
Waki' bin al-Jarrah narrated it from Abdullah bin Sa'id as mawquf (stopping at) Ibn Abbas (may Allah be pleased with them both), and it has also been narrated from Ibn Abbas through another, weak, route.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20077