Musannaf Ibn Abi Shayba 3:5669

Abu Mu'awiyah narrated to us, from al-A'mash, from Zayd ibn Wahb, from Abdullah, that he entered the mosque and there was a man praying while his hair was braided (tied up). When he finished, Abdullah said, "When you pray, do not braid your hair, for your hair prostrates with you, and you have a reward for every hair." The man said, "I fear that it will get dusty." He (Abdullah) said, "Its getting dusty is better for you."