Abu Hurairah reported,
A black man, or a black woman, used to sweep the mosque. When he died, the Prophet ﷺ asked about him, and the people said, "He has died."He said, "Why did you not inform me? Show me his grave," or he said, "her grave."So he came to the grave and prayed over him.
Azami 3108
Sahihعن أبي هريرة
أن رجلا أسود- أو امرأة سوداء- كان يقم المسجد، فمات فسأل النبي ﷺ عنه فقالوا: مات، قال: أفلا كنتم آذنتموني به دلوني على قبره- أو قال قبرها- فأتى قبره فصلى عليه.
Bukhari narrated it in the Book of Prayer (458), and Muslim in the Book of Funerals (956), both from the hadith of Hammad bin Zayd, from Thabit Bunani, from Abu Rafi, from Abu Hurairah, who related the same account. The wording given here is Bukhari's. Muslim added, "It was as though they had considered her matter (or his matter) insignificant." Then he said, "These graves are filled with darkness for their occupants, and Allah 
illuminates them by my prayer over them." The doubt as to whether it was a man or a woman comes from one of the narrators of the hadith. The predominant view is that it was a woman, as Hammad bin Zayd said in another narration, "I only think it was a woman," Bukhari (460). Some have named her Umm Mihjan, and she used to sweep the masjid of the Messenger of Allah ﷺ.