Maliks Muwatta Book 22, Hadith Number 8.

Yahya related to me from Malik from Talha ibn Abi al-Malik al-Ayli from al-Qasim ibn Muhammad ibn as-Siddiq from A’isha that the Messenger of Allah, may Allah bless him and grant him peace, said, “Whoever vows to obey Allah, let him obey Him. Whoever vows to disobey Allah, let him not disobey Him.”

Yahya said that he had heard Malik say, “The meaning of the statement of the Prophet, may Allah bless him and grant him peace, ‘Whoever vows to disobey Allah, let him not disobey Him’ is that for instance a man who vows that, if he speaks to such-and-such a person, he will walk to Syria, Egypt, or any other such things which are not considered as ibada, is not under any obligation by any of that, even if he did speak to the man or did break whatever it was he swore, because Allah does not demand obedience in such things. He should only fulfil those things in which there is obedience to Allah.”