Musnad Ahmad 27:1143

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Mansur, from 'Abdullah ibn Murrah, from Ibn 'Umar, who said: The Prophet, peace and blessings of Allah be upon him, forbade vowing, and he said: "It does not bring any good, and it is only a means by which something is extracted from the miser."