This would be a good place to start. It’s the same passage quoted earlier by @CelticWarlord and @CajunJoy65:
Then Peter came to him and asked, Lord, how often must I see my brother do me wrong, and still forgive him; as much as seven times? Jesus said to him, I tell thee to forgive, not seven wrongs, but seventy times seven (Matt 18:21-22).
Jesus follows that up with the parable of the Unforgiving Servant:
Here is an image of the kingdom of heaven; there was a king who resolved to enter into a reckoning with his servants, and had scarcely begun the reckoning, when one was brought before him who was ten thousand talents in his debt. He had no means of making payment; whereupon his master gave orders that he should be sold, with his wife and children and all that he had, and so the debt should be paid. With that the servant fell at his feet and said, Have patience with me, and I will pay thee in full. And his master, moved with pity for him, let the servant go and discharged him of his debt.
So the servant went out, and met with a fellow servant of his, who owed him a hundred pieces of silver; whereupon he caught hold of him and took him by the throat, and said, Pay me all thou owest me. His fellow servant went down on his knees in entreaty; Have patience with me, he said, and I will pay thee in full. But the other refused; he went away and committed him to prison for such time as the debt was unpaid.
The rest of the servants were full of indignation when they saw this done, and went in to tell their master what had happened. And so he was summoned by his master, who said to him, I remitted all that debt of thine, thou wicked servant, at thy entreaty; was it not thy duty to have mercy on thy fellow servant, as I had mercy on thee? And his master, in anger, gave him over to be tortured until the debt was paid. It is thus that my heavenly Father will deal with you, if brother does not forgive brother with all his heart.