Musnad Ahmad 27:826

'Abd al-Rahman narrated to us, from Sufyan, from Mansur, from 'Abdullah ibn Murrah, from Ibn 'Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) forbade vowing. He said: "It does not avert anything of the decree, but it is only a means by which something is extracted from the miser."