Abu Salih narrates: I was with Sayyidah Umm Salamah (may Allah be pleased with her) when a young relative of hers, whose hair reached his shoulders, came in to her. He stood up to pray and began blowing. She said: My son, do not blow, for I heard the Messenger of Allah (peace and blessings be upon him) say to a black slave of ours: "O Rabah, put dust on your face (that is, place your forehead on the ground)."
Its narrator Maymun Abu Hamzah is da'if (weak). Another hadith on this is narrated from Sayyiduna Zayd bin Thabit (may Allah be pleased with him) as marfu' (from the Prophet), and it is extremely weak.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3363