The Rebbe joyfully congratulates the recipient on the birth of a son, blessing that the bris take place on time and that he be raised in Torah, marriage, and good deeds with material and spiritual abundance. He emphasizes the joy associated with this mitzvah, citing halachic and Chassidic sources, and concludes with Mazel Tov.
https://www.torahrecordings.com/rebbe/004_igros_kodesh/adar/909