The Messenger of Allah (ﷺ) said: Two sheep which resemble each other are to be sacrificed for a boy and one for a girl.

Abu Dawud said: This is a sound tradition, and the tradition narrated by Sufyan is misunderstanding.
Sunan Abi Dawud Book 16, Hadith 49
← Previous Hadith I heard the Prophet (nay peace be upon him) say: Let the birds stay in their roosts. She said: I also heard him say: Two sheep are to be sacrificed for a boy and one for a girl, but it does you no harm whether they are male or female.

Sunan Abi Dawud Book 16, Hadith 48 Next Hadith → The Prophet (ﷺ) said: A boy is in pledge for his Aqiqah. Sacrifice is made for him on the seventh day, his head is shaved and is smeared with blood.

When Qatadah was asked about smearing with blood, how that should be done, he said: When you cut the head (i.e. throat) of the animal (meant for Aqiqah), you may take a few hair of it, place them on its veins, and then place them in the middle of the head of the infant, so that the blood flows on the hair (of the infant) like a threat. Then its head may be washed and shaved off.

Abu Dawud said: In narrating the word "is smeared with blood" (yudamma) there is a misunderstanding on the part of Hammam.

Abu Dawud said: Hammam has been opposed in narrating the words "is smeared with blood". This is misunderstanding of Hammam. They narrated he word "he is given a name (yusamma) and Hammam narrated it "is smeared with blood" (yudamma).

Abu Dawud said: This tradition is not followed.

Sunan Abi Dawud Book 16, Hadith 50