To someone who says “May Allah bless you”
To someone who says “May Allah bless you”: 1 supplication in Arabic with English translation and sources, from the Fortress of the Muslim (Hisn al-Muslim).
الدعاء لمن قال بارك الله فيك
وَفِيكَ بَارَكَ اللَّهُ.
And may Allah bless you too.
Fortress of the Muslim (Hisn al-Muslim), Sa‘id ibn ‘Ali al-Qahtani. MushafPlus translation made from the Arabic, not reviewed by a specialist.