Sayyiduna Ibn Abbas (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) said: "Whoever makes a vow and does not name it, his expiation for the vow is that of an oath; and whoever makes a vow that he is unable to fulfil, his expiation too is that of an oath."
Abu Dawud said: Waki' narrated it from Abdullah bin Sa'id bin Abi Hind as mawquf, stopping at Ibn Abbas (may Allah be pleased with them both). It has also been narrated as marfu' through another route from Bukayr bin al-Ashajj that is not strong.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19913