Bārakallahu laka wa bāraka `alayk, wa jazākallāhu khayra, wa razaqakallāhu mithlah, wa ajzala thawābak.
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May Allah bless you with His gift to you, and may you (the new parent) give thanks, may the child reach the maturity of years, and may you be granted its righteousness.

The reply of the person being congratulated is to say:

May Allah bless you, and shower His blessings upon you, and may Allah reward you well and bestow upon you its like and reward you abundantly