If a Muslim brother want to tell him that he loves him, he should say:
أُحِبُّكَ فِي اللَّهِ
Pronunciation
U'hibbuka fill-aah
Meaning
I love you for the sake of Allah. He will respond by saying:
Source: Hasan (Albani). Abu Dawud: 5125
Duas from IslamicAPI's Masnun Dua dataset (based on the Quran, hadith, Hisnul Muslim and Rahe Belayat). See the dataset