Event: Cersei Lannister - Secret Discovery (Political Wins)

Episode 5: Eastwatch at 12:57
Jaime explains to Cersei that it was Olenna Tyrell who had assassinated Joffrey, not Tyrion.

Jaime: "It was Olenna. She confessed before she died."
Cersei: "And this was before or after she drank the poison you so kindly provided her?"
Jaime: "After."
Cersei: "And you believed her?"
Jaime: "If you were Olenna, would you rather have seen your granddaughter married to Joffrey or Tommen? Which one would Margaery have been better able to control? Which one would have made Olenna the true ruler of the Seven Kingdoms? She was telling the truth."
(Secret Discovery)