Sayyiduna Abu Rafi' (may Allah be pleased with him) said: "The Messenger of Allah (peace and blessings be upon him) passed by me while I was in prostration and had tied up my hair, so he (peace and blessings be upon him) undid it and forbade me from that."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 987