Surah Al-Fath 48:4

هُوَ ٱلَّذِىٓ أَنزَلَ ٱلسَّكِينَةَ فِى قُلُوبِ ٱلْمُؤْمِنِينَ لِيَزْدَادُوٓا۟ إِيمَـٰنًا مَّعَ إِيمَـٰنِهِمْ ۗ وَلِلَّهِ جُنُودُ ٱلسَّمَـٰوَٰتِ وَٱلْأَرْضِ ۚ وَكَانَ ٱللَّهُ عَلِيمًا حَكِيمًا

It is He who sent down tranquility into the hearts of the believers that they would increase in faith along with their [present] faith. And to Allāh belong the soldiers of the heavens and the earth, and ever is Allāh Knowing and Wise.

Translation: Saheeh International

Tafsir

He it is Who sent down the spirit of Peace Divine reassurance into the hearts of the believers that they might add faith to their faith by way of their embracing of the legal stipulations of religion for every time He revealed one of these they would believe in it — including the command to struggle. …

Excerpt from Tafsir al-Jalalayn, commentary on 48:4, via Quran.com. The reader has the full commentary and 9 more tafsirs.