Mishkat al-Masabih 2601
Jabir reported God’s messenger as saying :
When the day of ‘Arafa comes God descends to the lowest heaven and praises them to the angels  saying, “Look at my servants who have come to me dishevelled, dusty and crying out from every deep valley. I call you to witness that I have forgiven them.” Then the angels say, “My Lord, so and so was being suspected of sin, also so and so and such and such a woman.” He said  that God who is great and glorious replies, “I have forgiven them.” God’s messenger said, “No day has more people set free from hell than the day of ‘Arafa.”

It is transmitted in Sharh as-sunna.
وَعَنْ جَابِرٍ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: 
" إِذَا كَانَ يَوْمُ عَرَفَةَ إِنَّ اللَّهَ يَنْزِلُ إِلَى السَّمَاءِ الدُّنْيَا فَيُبَاهِي بِهِمُ الْمَلَائِكَةَ فَيَقُولُ: انْظُرُوا إِلَى عِبَادِي أَتَوْنِي شُعْثًا غُبْرًا ضَاجِّينَ مِنْ كُلِّ فَجٍّ عَمِيقٍ أُشْهِدُكُمْ أَنِّي قَدْ غَفَرْتُ لَهُمْ فَيَقُولُ الْمَلَائِكَةُ: يَا رَبِّ فُلَانٌ كَانَ يُرَهَّقُ وَفُلَانٌ وَفُلَانَةُ قَالَ: يَقُولُ اللَّهُ عَزَّ وَجَلَّ: قَدْ غَفَرْتُ لَهُمْ ". قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «فَمَا مِنْ يَوْمٍ أَكْثَرَ عَتِيقًا مِنَ النَّارِ مِنْ يَوْمِ عَرَفَةَ» . رَوَاهُ فِي شرح السّنة