narrated salman bin 'amir ad-dabbi (radi-allahu 'anhu):

i heard allah's apostle (sallallahu 'alaihi wa sallam) 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 (sallallahu 'alaihi wa sallam) was asked about aqiqa and he ordered 2 sheep for a boy and one sheep for a girl and that is his tradition "sunna".)

bukhari vol. 7 : no. 380