How sound is the narration that states that Lady Umm Mihjan died and the Prophet spoke to her after her death, whereupon she informed him that the best of deeds is sweeping mosques? And who narrated it, and what is its تخريج (source and verification)?

It is established from the story of the mosque's female servant, as narrated by the two Sheikhs (Bukhari and Muslim), that a black woman used to clean the mosque. The Prophet, peace and blessings be upon him, inquired about her, and when he learned of her death, he prayed over her grave. As for the story of his asking her about the best deeds and her answer, its meaning came in an addition narrated by Abu Al-Sheikh Al-Asbahani and mentioned by Al-Suyuti and Al-Mundhiri. However, Siddiq Hassan Khan and Sheikh Al-Albani deemed it weak because it is mursal (lacks a companion in the chain of narration) or da'if mu'dal (a weak hadith with two or more consecutive narrators missing from the chain).