Zayd narrates: Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) passed by a man in prostration who had tied up his hair, so he undid his knot and let the hair loose, then waited until he had completed the prayer. Then Sayyiduna Abdullah (may Allah be pleased with him) said: "Your hair prostrates along with you, so do not tie it up, for you receive a reward for every single hair of it." The man said: I feared that dust would get on it. Sayyiduna Abdullah (may Allah be pleased with him) said: "If dust gets on it, that is better for you."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9300