It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "Whoever makes a vow and does not specify it, its expiation is like the expiation for an oath; whoever makes a vow to disobey Allah, its expiation too is like the expiation for an oath; whoever makes a vow that he is not able to fulfil, its expiation too is like the expiation for an oath; and whoever makes a vow that he is able to fulfil, let him fulfil it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12117