On the authority of Abu Hurayrah (may Allah be pleased with him), who said that the Messenger of Allah (PBUH) said:

The gates of Paradise will be opened on Mondays and on Thursdays, and every servant [of Allah] who associates nothing with Allah will be forgiven, except for the man who has a grudge against his brother. [About them] it will be said: Delay these two until they are reconciled; delay these two until they are reconciled.

It was related by Muslim (also by Malik and Abu Dawud).