هُوَ الَّذِي أَنزَلَ السَّكِينَةَ فِي قُلُوبِ الْمُؤْمِنِينَ لِيَزْدَادُوا إِيمَانًا مَّعَ إِيمَانِهِمْ ۗ وَلِلَّهِ جُنُودُ السَّمَاوَاتِ وَالْأَرْضِ ۚ وَكَانَ اللَّهُ عَلِيمًا حَكِيمًا
“He is the One Who sent down serenity upon the hearts of the believers so that they may increase even more in their faith. To Allah [alone] belong the forces of the heavens and the earth. And Allah is All-Knowing, All-Wise.”
This ayah is part of Surah al-Fath, revealed -- as this event's own significance note states -- "concerning this incident" (Q48:1), the Treaty of Hudaybiyyah.