Musannaf Ibn Abi Shayba 7:286

Ghundar narrated to us, from Shu'bah, from 'Abdullah ibn Murrah, from Ibn 'Umar, from the Prophet (peace and blessings of Allah be upon him), that he forbade vowing. He said: "It does not bring about any good; it is only a means by which something is extracted from the miser."