5x22 - Dragon Ball Z: Dragon Box

Cell is surprised by his own power, far beyond his perfect form, and takes delight in it. In order to bring everything to a close, Cell builds up enough Ki to destroy the solar system, and is about to release a super-huge Kamehame-Ha. As Gohan is overwhelmed by this tremendous Ki, he hears the voice of Goku in his head, assisted by Kaio. Gohan, encouraged by his father's voice, then counters with his own super Kamehame-Ha.