If a man vow a vow unto the Lord , or swear an oath to bind his soul with a bond; he shall not break his word, he shall do according to all that proceedeth out of his mouth. Numbers 30:2 KJV
A man is only as good as his word. God watches over his word to make sure it does what He says.
Then said the Lord unto me, Thou hast well seen: for I will hasten my word to perform it. Jeremiah 1:12 KJV
And being fully persuaded that, what he had promised, he was able also to perform. Romans 4:21 KJV
He expects us to do the same.
. . . he honoureth them that fear the Lord. He that sweareth to his own hurt, and changeth not. Psalms 15:4 KJV