Musnad Ahmad 3:19

Abdullah narrated to us, Shayban Abu Muhammad narrated to us, Mahdi ibn Maymun narrated to us, Muhammad ibn Abdullah ibn Abi Ya’qub narrated to us, from al-Hasan ibn Sa’d, from Rabah, and he mentioned the hadith. He said: So I raised them to the Commander of the Faithful, Uthman ibn Affan, and he said: "Indeed, the Messenger of Allah, may Allah bless him and grant him peace, decreed that the child belongs to the bed..." and he mentioned the like of it.