How sound is the Hadith that mentions a king, entrusted with the ears of creation, standing at the Prophet's grave, conveying to him the salutations of those who invoke blessings upon him, along with their names and their fathers' names, and that Allah has guaranteed to respond with ten blessings for each salutation?

The hadith narrated from Ammar ibn Yasir (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: "Indeed, Allah has appointed an angel to my grave who has been given the hearing of all creation. No one sends blessings upon me until the Day of Resurrection except that he conveys it to me by his name and his father's name: 'This is so-and-so the son of so-and-so who has sent blessings upon you'," is a very weak hadith due to Nu'aym ibn Damdam and Imran ibn Humairi. It has been weakened by scholars.

Other authentic hadiths, however, confirm that the Prophet (peace and blessings of Allah be upon him) is informed of the salutations of his Ummah and responds to them, such as: "There is no one who sends salutations upon me except that Allah returns my soul to me so that I may return his salutation," and "Increase your blessings upon me, for Allah has appointed an angel near my grave. When a man from my Ummah sends blessings upon me, that angel says to me: 'O Muhammad, so-and-so the son of so-and-so has just sent blessings upon you.'"

As for the addition: "And my Lord, the Blessed and Exalted, has guaranteed me that I will return to him ten for every prayer," there is no evidence for it. What is better than that has been reported regarding the virtue of sending blessings upon the Prophet (peace and blessings of Allah be upon him), which is that Allah Almighty sends blessings upon whoever sends blessings upon him ten times, as in the hadith: "Whoever sends blessings upon me once, Allah sends blessings upon him ten times."