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بَارَكَ اللَّهُ لَكَ فِي الْمَوْهُوبِ لَكَ, وَشَكَرْتَ الْوَاهِبَ, وَبَلَغَ أَشُدَّهُ, وَرُزِقْتَ بِرَّهُ. وَيَرُدُّ عَلَيْهِ الْمُهَنَّأُ فَيَقُولُ: بَارَكَ اللَّهُ لَكَ وَبَارَكَ عَلَيْكَ, وَجَزَاكَ اللَّهُ خَيْراً, وَرَزَقَكَ اللَّهُ مِثْلَهُ, وَأَجْزَلَ ثَوَابَكَ
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
Source: An-Nawawi Al-Azkar 349.