Hadith 5:116
He said that the Prophet used to fast on Mondays and Thursdays, and when someone remarked on this to him he said, “On Monday and Thursday God forgives every Muslim except two who are at variance, for He says they must be left till they make peace with one another.” Ahmad and Ibn Majah transmitted it.
وَعَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: كَانَ يَصُومُ يَوْمَ الِاثْنَيْنِ وَالْخَمِيسِ. فَقِيلَ: يَا رَسُولَ اللَّهِ إِنَّكَ تَصُومُ يَوْمَ الِاثْنَيْنِ وَالْخَمِيسِ. فَقَالَ: " إِنَّ يَوْمَ الِاثْنَيْنِ وَالْخَمِيسِ يَغْفِرُ اللَّهُ فِيهِمَا لِكُلِّ مُسْلِمٍ إِلَّا ذَا هَاجِرَيْنِ يَقُولُ: دَعْهُمَا حَتَّى يصطلحا " رَوَاهُ أَحْمد وَابْن مَاجَه
Original Reference: Mishkat al-Masabih 2073