John Gill Commentary Numbers 23:26

"But Balaam answered and said unto Balak, Told not I thee, saying, All that Jehovah speaketh, that I must do?" — Numbers 23:26 (ASV)

But Balaam answered and said to Balak, told not I you
He appeals to him for his honesty and faithfulness, for honest and faithful he would be thought to be, both to God and man; that when he first met him, he plainly told him what he must expect: from him:

saying, all that the Lord speaks, that I must do ;
which was very true, he was obliged to do as he had bid him, and speak what he had said to him, though it was sore against his will; he would fain both have spoken and done otherwise, if he might have been permitted.