John Gill Commentary Numbers 14:28

"Say unto them, As I live, saith Jehovah, surely as ye have spoken in mine ears, so will I do to you:" — Numbers 14:28 (ASV)

Say unto them, [as truly as] I live, says the Lord
The form of an oath, as in (Numbers 14:21);

as you have spoken in my ears, so will I do to you;
what they had wished for, and expressed in the hearing of the Lord, he threatens them should be their case.