48:4

(48:4) He is the One who sends down the Tranquility into the hearts of those who acknowledge, so that they may increase in acknowledgement along with their present acknowledgement. To God belongs the soldiers of the heavens and the earth, and God is knowledgeable, wise.

Connected

(48:18) God is pleased with those who acknowledge who pledged allegiance to you under the tree. He thus knew what was in their hearts, so He sent down the Tranquility upon them, and rewarded them with a near victory.