Du'a for marital harmony

اللَّهُمَّ أَلِّفْ بَيْنَ قُلُوبِنَا وَأَصْلِحْ ذَاتَ بَيْنِنَا وَاهْدِنَا سُبُلَ السَّلَامِ وَنَجِّنَا مِنَ الظُّلُمَاتِ إِلَى النُّورِ

Allahumma allif bayna qulubina wa aslih dhata baynina wahdina subulas-salam wa najjina minadh-dhulumati ilan-nur

O Allah, reconcile our hearts, resolve our affairs, guide us to the paths of peace, and save us from darkness to the light.

Sunan Abī Dāwūd 969 (part)