Hadeeth Qudsi 20:

On the authority of AbuHurayrah (may Allah be pleased with him), who said that the Messengerof Allah   said:

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

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