When he let Freiza know about the Dragon Balls on Namek.
Vegeta and Nappa find out about the Earth Dragon Balls by listening to a transmission on Raditz's scouter. They also learn that Goku and his crew have the ability to suppress their power levels, thus making the scouters all but useless, as referenced in the infamous "over 9000!!!" video. Despite this, Vegeta decides to bring two fully functional, transmitting scouters with him on his secret mission to gather the Dragon Balls, wish for immorality, and commit treason against his master Freiza. During the battle, Earth's Dragon Balls die along with Piccolo, but the existence of a separate set of balls on Planet Namek is revealed. You seriously didn't think Freiza also had access to scouter transmission logs?!?!