Al-Hakam and Talhah bin Musarrif narrate: Ma'qil bin Sinan came to Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) and asked him about a man who makes a vow but does not specify anything in the vow. He said: "He should free a slave."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9165