let bygones be bygones

From bygone (“event that happened in the past”).

• ""Never mind, I'll give you a hundred dollars for it," said the king; "you did me out of horse and saddle the other day, and the bridle too, but I'll let bygones be bygones, if I get the pot.""
• "To intrude as little of his personality as possible upon a gay event with which that personality could show nothing in keeping, he decided not to make his appearance till evening—when stiffness would have worn off, and a gentle wish to let bygones be bygones would exercise its say in all hearts."
• "You'll find, when you come back, I promise you, Kit, that everyone is willing to let bygones be bygones, and that you can make a fresh start."
• "But I ain't got no quarrel witcha. Just let bygones be bygones."

• forgive and forget

• let sleeping dogs lie
• water under the bridge