L examines evidence picked up by Watari, but then requests Chief Yagami & the others to assume Light is Kira if he dies in the next few days. As Ryuzaki thinks, he decides to make a wager to take down Kira & the 2nd Kira.
This is from Episode 15. Note: I do not own Death Note, it belongs to: Tsugumi Ohba, Takeshi Obata & licensed to Viz Media (North American distribution). YouTube – Inform me if you wish for this removed.