Musannaf Ibn Abi Shayba 3:5665

Abu Usama narrated to us, from Shu'bah, from al-Mukhawwal, from Abu Sa'id, from Abu Rafi', who said: The Prophet (peace be upon him) passed by me while I was prostrating, and I had tied up my hair. So he loosened it, or he said: He forbade me from it.