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).