Sunnan Abu Dawud Collection :: Book Number 21
21.3316
Narrated Abdullah ibn Abbas: The Prophet (peace_be_upon_him) 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.