What is the legal evidence that vowing to other than God is major shirk?

A vow (nadhr) is an act of worship that may not be directed to anyone but Allah. Directing it to other than Him is major shirk (polytheism), due to the veneration of the one for whom the vow is made that is present in the heart of the one making the vow, and the belief that he is worthy of worship, such as believing in his power to cause harm or benefit. This is supported by the fact that a vow is an act of worship valid only for Allah, and whoever directs it to other than Him has committed shirk. Some scholars are of the view that making a vow to other than Allah, without veneration, or belief in harm or benefit, or causality, but rather to venerate the act of vowing itself, is considered a reprehensible innovation (bid'ah) and a cause of shirk, though it does not reach the level of major shirk. Its status in this regard is similar to swearing by other than Allah: if it is done out of veneration, it is major shirk, but if not, it is minor shirk.