In the Quran there is no mention of witnesses for marriage but Allah made it compulsory for divorce to have two witnesses at the time the divorce is being recited

Holy Quran 65:2
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فَإِذَا بَلَغْنَ أَجَلَهُنَّ فَأَمْسِكُوهُنَّ بِمَعْرُوفٍ أَوْ فَارِقُوهُنَّ بِمَعْرُوفٍ وَأَشْهِدُوا ذَوَيْ عَدْلٍ مِّنكُمْ وَأَقِيمُوا الشَّهَادَةَ لِلَّهِ ۚ ذَٰلِكُمْ يُوعَظُ بِهِ مَن كَانَ يُؤْمِنُ بِاللَّهِ وَالْيَوْمِ الْآخِرِ ۚ وَمَن يَتَّقِ اللَّهَ يَجْعَل لَّهُ مَخْرَجًا

And when they have [nearly] fulfilled their term, either retain them according to acceptable terms or part with them according to acceptable terms. And bring to witness two just men from among you and establish the testimony for [the acceptance of] Allah. That is instructed to whoever should believe in Allah and the Last day. And whoever fears Allah – He will make for him a way out