بَارَكَ اللَّهُ لَكَ فِي الْمَوْهُوبِ لَكَ وَ شَكَرْتَ الْوَاهِبَ , وَبَلَغَ أَشُدَّهُ وَرُزِقَْتَ بِِرَّهُ

145. Baarakallaahu laka fil-mawhoobi laka, wa shakartal-waahiba , wa balagha 'ashuddahu, wa ruziqta birrahu.

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:

باَرَكَ اللَّهُ لَكَ وَبارَكَ عَلَيْكَ وَجَزَاكَ اللّهُ خَيْراً وَرزََقَكَ اللَّهُ مِثْلُهُ وَ أَجْزَلَ َثَوابَكَ

Baarakallahu laka wa baaraka 'alayka, wa jazaakallaahu khayran, wa razaqakallaahu mithlahu, wa 'ajzala thawaabaka.

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

Reference:

An-Nawawi, Kitdbul-'Athkarp. 349, and Sahihul-'Athkar2/7l3 by Saleem Al-Hilali.