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 al-Mujtaba (1282), and in al-Yawm wal-Laylah (66), and Imam Ahmad (3666), and Ibn Hibban graded it sahih (914), and Hakim (2/421), all of them through various routes from Sufyan, from Abdullah bin Saaib, from Zazan, from Abdullah, who mentioned it. Its chain of transmission is sahih. See further, the collected chapters on faith in the angels.Ibn Umar, whenever he returned from a journey, would come to the grave and say, "Peace be upon you, O Messenger of Allah. Peace be upon you, O Abu Bakr. Peace be upon you, O my father." In another version, he would begin at the grave of the Messenger of Allah ﷺ, invoking blessings and peace upon him and praying for him, without touching the grave. Bayhaqi narrated it in al-Kubra (5/245) and in al-Sughra (1749, according to my numbering).Imam Malik, Ahmad, and Shafii said that one says this while facing the chamber. He does not stand at the grave to pray for himself, because none of the companions used to do so, but rather they would face the qibla.On this subject there is also a report from Ali bin Husayn, from his father, from his grandfather, from the Messenger of Allah ﷺ, who said, "Do not make my grave a festival site, nor make your houses graves, for your greetings reach me wherever you are." Abu Yaala narrated it (469) from Abu Bakr bin Abu Shayba, who said, Zayd bin Hubab narrated to us, who said, Jafar bin Ibrahim, from the descendants of Dhul-Janahayn, narrated to us, who said, Ali bin Umar narrated to us, from his father, from Ali bin Husayn, who mentioned it. This chain includes Ali bin Umar, who is the son of Husayn bin Ali bin Abu Talib al-Hashimi. No one deemed him reliable except Ibn Hibban, and in Taqrib he is described as "obscure."On this subject there is also a report from Hasan bin Ali bin Abu Talib, who said, The Messenger of Allah ﷺ said, "Pray in your houses, do not turn them into graves, and do not make my house a festival site. Send blessings and greetings upon me, for your blessings and greetings reach me wherever you are." Abu Yaala narrated it (6761) from Musa bin Muhammad bin Hayan, who said, Abu Bakr Hanafi narrated to us, who said, Abdullah bin Nafi narrated to us, who said, Ala bin Abdul-Rahman told me, I heard Hasan bin Ali bin Abu Talib mention it. This chain includes Abdullah bin Nafi, who is Sayigh. When he narrated from memory he would err, but when he narrated from a written text he was accurate, and here he did not narrate from a written text, so he may well have erred, for this hadith is known to be from Abu Hurairah, as mentioned earlier, and it was for this reason that Haythami criticized it in Majma (2/247).As for the hadiths narrated about visiting the grave of the Prophet ﷺ, such as his saying, "Whoever performs hajj and then visits my grave after my death is as if he visited me during my life," and such as, "Whoever performs hajj to the House and does not visit me has been unkind to me," and such as, "Whoever performs the hajj of Islam, visits my grave, goes on a military expedition, and prays upon me in Bayt al-Maqdis, Allah will not question him about what He made obligatory upon him," and such as, "Whoever visits my grave, my intercession becomes obligatory for him," and such as, "Whoever visits me after my death is as if he visited me while I was alive," none of these hadiths, nor others like them, are sahih. See their detailed sourcing in Minna Kubra (4/401–407).