Sayyidah Asma bint Yazid (may Allah be pleased with her) narrates this saying of the Prophet (peace and blessings be upon him): "The aqiqah is a due right, in which two equal goats shall be slaughtered on behalf of a boy and one goat on behalf of a girl."
Ahmad and al-Tabarani in al-Kabir recorded it, and its narrators have been used as evidence (muhtajj bihim).