The دعاء for congratulating on a newborn is: 'Barakallahu laka fil-mawhubi lak, wa shakarta al-Wahib, wa balagha ashuddahu, wa ruziqta birrahu' — May Allah bless you in what He has given you, may you give thanks to the Giver, may the child reach maturity, and may you be granted their righteousness.
بَارَكَ اللَّهُ لَكَ فِي الْمَوْهُوبِ لَكَ، وَشَكَرْتَ الْوَاهِبَ، وَبَلَغَ أَشُدَّهُ، وَرُزِقْتَ بِرَّهُ
Barakallahu laka fil-mawhubi lak, wa shakarta al-Wahib, wa balagha ashuddahu, wa ruziqta birrahu.
Meaning: May Allah bless you in what He has given you, may you give thanks to the Giver, may the child reach full maturity, and may you be granted their righteousness.
Source: Musnad Ahmad #16522Congratulating parents on a newborn is a سنة that strengthens community bonds. النبي (صلى الله عليه وسلم) also مستحب giving the child a good name, performing aqiqah on the seventh day, shaving the head, and giving sadaqah equal to the weight of the hair in silver. A righteous child is among the greatest blessings a person can receive.
بَارَكَ اللَّهُ لَكَ فِي الْمَوْهُوبِ لَكَ، وَشَكَرْتَ الْوَاهِبَ، وَبَلَغَ أَشُدَّهُ، وَرُزِقْتَ بِرَّهُ
This is the congratulation taught by the early scholars when a child is born. Al-Nawawi and Ibn al-Qayyim documented this in their works as the recommended form of tahni'ah (congratulation) for a newborn.