The messenger of Allah (may Allah’s peace and blessings be upon him) commanded them to slaughter two similar sheep for a boy and one sheep for a girl (at their birth). [At-Tirmidhi]

"«أمر رسول الله -صلى الله عليه وسلم- أمر أن يُعَقَّ عن الغُلَامِ شَاتَانِ مُكَافِئَتَانِ، وعن الْجَارِيَةِ شَاةٌ». [صحيح.] - [رواه الترمذي وأحمد.]"