Zayd bin Wahb narrates: Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) passed by a man in prostration whose hair was braided up, so he undid it. When the man finished the prayer, Sayyiduna Abdullah (may Allah be pleased with him) said to him: "Do not tie up your hair, for your hair also prostrates, and you receive a reward for every single hair." He said: I only tied it so that dust would not get on it. He said: "If dust gets on it, that is better for you."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9299