Narrated Ibn 'Abbas (RA):

The Prophet (ﷺ) returned his daughter Zainab (RA) to [her husband] Abul-'Aas bin ar-Rabi' (RA) based upon the first marriage, after six years (of separation), and he did not perform a new marriage. [Reported by Ahmad and al-Arba'a except an-Nasa'i. Ahmad and al-Hakim graded it Sahih (authentic)].

وَعَنِ ابْنِ عَبَّاسٍ ‏- رَضِيَ اَللَّهُ عَنْهُمَا‏- قَالَ : { رَدَّ اَلنَّبِيُّ ‏- صلى الله عليه وسلم ‏-اِبْنَتَهُ زَيْنَبَ عَلَى أَبِي الْعَاصِ بْنِ اَلرَّبِيعِ , بَعْدَ سِتِّ سِنِينَ بِالنِّكَاحِ اَلْأَوَّلِ , وَلَمْ يُحْدِثْ نِكَاحًا } رَوَاهُ أَحْمَدُ , وَالْأَرْبَعَةُ [1]‏ إِلَّا النَّسَائِيَّ , وَصَحَّحَهُ أَحْمَدُ , وَالْحَاكِمُ [2]‏ .‏