It is reported about Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that he passed by a man who was in prostration with his hair tied up in a knot. Sayyiduna Abdullah (may Allah be pleased with him) undid it. When the man finished his prayer, Sayyiduna Abdullah (may Allah be pleased with him) said to him: "Do not tie your hair in a knot, for the hair also prostrates, and you will receive a reward for every single hair." The man said: "I only tied it up so that it would not get covered in dust." Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "If it gets covered in dust, that is better for you."
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir; its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2759