Reference : Hisn al-Muslim 204

وَفيكَ بارَكَ الله
Wafīka bārakallāh.

And may Allah bless you.

Reference:
Ibn As-Sunni, p. 138, (no. 278). See also Ibn Al-Qayyim, Al-Wdbil 
As-Sayyib, p. 304, with checking by Basheer Muhammad ?Uyoon.

Chapter: To someone who says "May Allah bless you"