Deuteronomy 26:16;
16. This day the Lord thy God hath commanded thee to do these statutes and judgments: thou shalt therefore keep and do them with all thine heart,and with all thy soul.
17. Thou hast avouched the Lord this day to be thy God, and to walk in his ways,and to keep his statutes, and his commandments, and his judgments, and to hearken unto his voice.
18. And the Lord hath avouched thee this day to be his peculiar people, as he hath promised thee, and that thou shouldest keep all his commandments.
19: And to make thee high above all nations which he hath made, and in honour; and that thou mayest be an holy people unto the Lord thy God, as he hath spoken.
AVouch - to state Firmly, Positively, or Assuredly.....

Isaiah 28:11-12.

11. For with stammering lips and another tongue will he speak to this people.

12. To whom he said, This is the rest wherewith ye may cause the weary to rest; and this is the refreshing: yet they would not hear.