Also: Take him at his word; Word of honor
Meaning: A solemn affirmation of intention, without an oath.
Matthew 5:37
But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.
James 5:12
But above all things, my brethren, swear not, neither by heaven, neither by the earth, neither by any other oath: but let your yea be yea; and your nay, nay; lest ye fall into condemnation.
2 Cor. 1:17-20
When I therefore was thus minded, did I use lightness? or the things that I purpose, do I purpose according to the flesh, that with me there should be yea yea, and nay nay? [18] But as God is true, our word toward you was not yea and nay. [19] For the Son of God, Jesus Christ, who was preached among you by us, even by me and Silvanus and Timotheus, was not yea and nay, but in him was yea. [20] For all the promises of God in him are yea, and in him Amen, unto the glory of God by us.