Dragon Ball Z - episode 31 - 19 Dec, 1989

Thanks to the Kaio-ken x3 technique, Goku has the upper hand against Vegeta, which infuriates the Saiyan Prince. He decides that if he can't beat Goku, he will destroy the Earth, and flies high up in the air and fires his Galick Gun attack. Goku unleashes the Kamehameha wave, and the two energy blasts meet in midair. Goku and Vegeta pour all of their energy into this attack, and after a long struggle, Vegeta's blast is pushing Goku's back, even with Goku using the Kaio-ken x3 attack. With no choice left, Goku uses a Kaio-ken x4 attack, and with the extra power, he is able to blast Vegeta into space. However, Vegeta is able to recover, enraged that Goku's power is superior to his. Vegeta decides to become a Great Ape for his rematch with Goku. The Saiyan Prince starts searching for the full moon, but cannot find it, thanks to Piccolo having destroyed it before the Saiyans' arrival. Thinking Goku must have blown up the moon just to prevent him from transforming, Vegeta returns to Earth where Goku is completely spent and claims to have a trick up his sleeve. He tells Goku the origin of how Saiyans transform into Great Apes through Blutz Waves: the power of a planets moon creates the Blutz Waves, and he can create and launch his own artificial moon into the sky in the event a planet has no moon. Absorbing the Blutz Waves reflecting from the artificial moon, Vegeta begins to transform into a Great Ape as Goku looks on in horror. Meanwhile, Krillin and Gohan feel the tremendous surge in energy coming from the battlefield, and decide to head back. (Source: Wikipedia)