The original video performed by hitomi (under the name Minami Hokuto) that inspired Magical Girl Lyrical Nanoha. It is from Triangle Heart ~Lyrical Toy Box~, a merchandise CD released on June 29, 2001 for the eroge video game Triangle Heart 3.
The CD contains a large amount of fan material, including a number of visual novel-like stories called "mini-scenarios". One of them, entitled "Magical Girl Lyrical Nanoha", cast Nanoha Takamachi (a minor character in the TH3 canon) as a magical girl and served as a prototype for the Magical Girl Lyrical Nanoha anime series.