The Messenger of Allah ﷺ said, "Allah has angels who roam the earth, conveying to me the greetings of peace from my ummah."
قال رسول الله ﷺ: إن الله ملائكة سياحين في الأرض يبلغون من أمتي السلام.
Nasai narrated it in Mujtaba (1282), and in Al-Yawm wal-Laylah (66), through various routes from Sufyan, from Abdullah bin Saaib, from Zazan, from Abdullah, and he related the same. Imam Ahmad narrated it (3666), and Ibn Hibban graded it sahih (914), as did Hakim (2/421), and Hafiz bin Qayyim also graded it sahih in Jala Afham (p. 54).As for what was narrated from Ammar bin Yasir, who said that the Messenger of Allah ﷺ said, "Allah has appointed an angel over my grave to whom He has given the hearing of all creatures, so no one will send blessings upon me until the Day of Resurrection except that this angel conveys it to me by name, saying, 'This is so-and-so, son of so-and-so, who has sent blessings upon you'," this is daif. Bazzar narrated it, in Kashf Astar (4/47), and Abu Shaykh, through the route of Nuaym bin Damdam, who said, Ibn Humayri narrated to us, from Ammar bin Yasir, and he related it. In it is Nuaym bin Damdam, about whom Haythami said in Al-Majmu (10/162), "Some scholars declared him daif. And Ibn Humayri, whose name is Imran, Bukhari said of him, 'his hadith is not corroborated,' and the author of Al-Mizan (Dhahabi) said, 'he is unknown.'"I say, Bukhari mentioned this in Tarikh al-Kabir (6/416) through this same route from Imran and said, "It is not corroborated."Also on this subject is what was narrated from Abu Bakr, as a saying attributed to the Prophet, "Increase your blessings upon me, for Allah has appointed an angel to be with me at my grave, and whenever a man from my nation sends blessings upon me, that angel says to him, 'O Muhammad, so-and-so, son of so-and-so, has just sent blessings upon you.'" Daylami narrated it in Musnad Firdaws. Sakhawi mentioned it in Al-Qawl Badi (p. 117) and said, "There is weakness in its chain."Likewise, what was narrated from Abu Umama, as a saying attributed to the Prophet, is not sahih, "Whoever sends one blessing upon me, Allah sends ten blessings upon him, and an angel is appointed to convey it to me." Tabarani narrated it (8/158), and in it is Musa bin Umayr Qurashi, the blind, who is extremely daif, as Haythami said in Al-Majmu (10/162).What is actually sahih regarding this is, "Whoever sends one blessing upon me, Allah sends ten blessings upon him," from the hadith of Abu Hurairah, narrated by Muslim in his Sahih (408), without mention of the angels.Likewise, what was narrated from Abdul-Rahman bin Awf, from the Prophet ﷺ, who said, "Jibreel came to me and gave me glad tidings, saying, 'Allah
says, whoever sends blessings upon you, I send blessings upon him, and whoever sends greetings of peace upon you, I send greetings of peace upon him,' so I prostrated to Allah
in gratitude," is not sahih. Imam Ahmad narrated it (1664), and in it is Abdul-Wahid bin Muhammad bin Abdul-Rahman bin Awf, from Abdul-Rahman bin Awf, and he related it. Abdul-Wahid was not deemed reliable by anyone except Ibn Hibban, and moreover it is not established that he heard directly from his grandfather Abdul-Rahman bin Awf. See its detailed sourcing in the chapter on the prostration of gratitude.As for the sahih hadiths on the merit of sending blessings upon the Prophet ﷺ, refer to them in their respective places.