Hadith 20, 40 Hadith Qudsi
On the authority of Abu Hurayrah (may Allah be pleased with him), who said that the Messenger of Allah (ﷺ) 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).
عَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ ،أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: 

" تُفْتَحُ أَبْوَابُ الْجَنَّةِ يَوْمَ الِاثْنَيْنِ، وَيَوْمَ الْخَمِيسِ، فَيُغْفَرُ لِكُلِّ عَبْدٍ لَا يُشْرِكُ بِاللَّهِ شَيْئًا، إِلَّا رَجُلًا كَانَتْ بَيْنَهُ وَبَيْنَ أَخِيهِ شَحْنَاءُ، فَيُقَالُ: (1) أَنْظِرُوا (2) هَذَيْنِ حَتَّى يَصْطَلِحَا، أَنْظِرُوا هَذَيْنِ حَتَّى يَصْطَلِحَا، أَنْظِرُوا هَذَيْنِ حَتَّى يَصْطَلِحَا" 
رواه مسلم (وكذلك مالك وأبو داود)