Sayyiduna Uqbah bin Amir (may Allah be pleased with him) said: I bear witness that I heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever makes a vow but does not name it, his expiation is the expiation of an oath."
(b) Sayyiduna Uqbah bin Amir (may Allah be pleased with him) reports from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "The expiation for a vow is the expiation of an oath."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19912