Arena Football League
Jim Foster, a promotions manager with the National Football League, conceived of indoor football while watching an indoor soccer match at Madison Square Garden in 1981. While at the game, he wrote his idea on a 9x12 envelope, with sketches of the field and notes on gameplay. He presented the idea to a few friends at the NFL offices, where he received praise and encouragement for his concept. After solidifying the rules and a business plan, and supplemented with sketches by a professional artist, Foster presented his idea to various television networks. He reached an agreement with NBC for a "test game".

Q: What was Jim Foster viewing when he came up with the idea for arena football?
an indoor soccer match