Abu Dawud Book 15, Hadith Number 3316.

Narated By Abdullah ibn Abbas : The Prophet (PBUH) said: If anyone takes a vow but does not name it, its atonement is the same as that for an oath, if anyone takes a vow to do an act of disobedience, its atonement is the same as that for an oath, if anyone takes a vow he is unable to fulfil, its atonement is the same as that for an oath, but if anyone takes a vow he is able to fulfil, he must do so.