Sayyidah Umm Atiyyah (may Allah be pleased with her) narrates that they arranged the hair of the daughter of the Prophet (peace and blessings be upon him) into three plaits, and she said: We undid it and washed it. Imam al-Bukhari (may Allah have mercy on him), in his Sahih, from Ahmad and Ibn Wahb (may Allah have mercy on them), added these words: "Then they made it into three plaits."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6769