Sforno on Deuteronomy 18:21/ספורנו על דברים י״ח:כ״א

1?איכה נדע את הדבר; when the authentic prophet commands a specific commandment such as when Elijah sacrificed on a private altar, something that had been forbidden for hundreds of years, or when Joshua waged war against Jericho on the Sabbath with all the violations this involved. (Kings I 18, and Joshua 6)