It is narrated from Sayyidah Umm Bani Kurz al-Ka'biyyah (may Allah be pleased with her) that she asked the Messenger of Allah (peace and blessings be upon him) about the aqiqah. He (peace and blessings be upon him) said: "Upon a boy, two equivalent sheep, and upon a girl, one sheep." (The narrator said:) I asked: "What does 'equivalent' mean?" He said: "That the two be alike."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21862