Or HaChaim on Numbers 13:1/אור החיים על במדבר י״ג:א׳

1 וידבר ה׳..אל משה לאמור, G'd spoke to Moses to say: Why was the word לאמור needed here, seeing the instructions were given only to Moses? Perhaps the Torah wrote לאמור to indicate that Moses had permission to tell the people that he had received the instructions to despatch spies from G'd. [The author is always at pains to prove that Moses had no right to relay anything G'd had told him unless especially told to do so; he bases himself on Yuma 4. Ed.] The Torah may also have wanted to prevent the impression being created that Moses initiated the idea of sending out spies and that he was of the same mind as the people in this respect. By telling the people that he had been commanded to select the spies he had made it plain that he would not have done so unless G'd had commanded him. The word לאמור was especially necessary in view of our sages' interpretation of the word לך meaning לדעתך, (verse 2) "in accordance with your understanding but not because you have to." In other words, G'd gave permission for the spies to go but He certainly did not command the expedition. It may have been hoped that when Moses told the people how he had been told by G'd to send the spies that they would reconsider and understand that their request to send spies revealed a lack of faith in G'd.