Muwatta Malik, Book 47, Hadith 17 (USC-MSA)

Yahya related to me from Malik from Suhayl ibn Abi Salih from his father from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, "The doors of the Garden are opened on Monday and Thursday. Every muslim slave who does not associate anything with Allah is forgiven except for the man who has enmity between him and his brother. It is said, 'Leave these two until they have made a reconciliation. Leave these two until they have made a reconciliation.' "

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ سُهَيْلِ بْنِ أَبِي صَالِحٍ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ تُفْتَحُ أَبْوَابُ الْجَنَّةِ يَوْمَ الاِثْنَيْنِ وَيَوْمَ الْخَمِيسِ فَيُغْفَرُ لِكُلِّ عَبْدٍ مُسْلِمٍ لاَ يُشْرِكُ بِاللَّهِ شَيْئًا إِلاَّ رَجُلاً كَانَتْ بَيْنَهُ وَبَيْنَ أَخِيهِ شَحْنَاءُ فَيُقَالُ أَنْظِرُوا هَذَيْنِ حَتَّى يَصْطَلِحَا أَنْظِرُوا هَذَيْنِ حَتَّى يَصْطَلِحَا ‏" ‏ ‏.‏

sunnah.com

also referenced under: Book 47, Hadith 1652 (In-Book)