Sahih Al Bukhari – Book of Sacrifice On Occasion Of Birth (`Aqiqa)
Volumn 007, Book 066, Hadith Number 380.
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Narated By Salman bin 'Amir Ad-Dabbi : I heard Allah's Apostle saying, "'Aqiqa is to be offered for a (newly born) boy, so slaughter (an animal) for him, and relieve him of his suffering." (Note: It has been quoted in Fateh-AL-Bari that the majority of the Religious Scholars agrees to the Hadith narrated in Sahih At-TlRMlZY that the Prophet was asked about Aqiqa and he ordered 2 sheep for a boy and one sheep for a girl and that is his tradition "SUNNA".)