Sayyiduna Samurah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him): "Every child is held in pledge for his aqiqah; on the seventh day an animal is slaughtered on his behalf, his head is shaved and he is smeared with blood." In the narration of al-Hawdi there is the addition that Qatadah (may Allah have mercy on him) was asked about applying the blood, and he said: When the aqiqah animal is slaughtered, take some of its wool, hold it against its veins to take the blood, then place it on the crown of the child's head so that the blood runs down like a thread. Then the head is washed and shaved.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19290