We say: if you send salaam upon him from the farthest ends of the earth, your salaam will reach him, because Allaah has appointed angels who travel about the earth, and if anyone sends salaams upon the Messenger (peace and blessings of Allaah be upon him), they convey that salaam to the Messenger (peace and blessings of Allaah be upon him). So if we say now, “O Allaah, send blessings and peace upon the Messenger of Allaah,” our salaam will be transmitted to him. In prayer we say, “Al-salaamu ‘alayka ayyuha’l-nabiyyu wa rahmat-Allaahi wa barakaatuhu (Peace be upon you, O Prophet, and the mercy of Allaah and His blessings),” and the salaam is transmitted to him…
I have heard some people saying in Madeenah, “My father asked me to give his salaams to the Messenger,” but this is wrong. The Messenger (peace and blessings of Allaah be upon him) is not alive so that the salaams of a living person may be passed on to him. If your father sends salaams to the Messenger, the salaam is conveyed by those who are more able than you to convey it and are more trustworthy than you too, namely the angels.
So there is no need for that, and we say: You are where you are, wherever you are on earth you can say, “Peace be upon you, O Prophet,” and it will reach him faster and more reliably than that.
Majmoo’ Fataawa al-Shaykh Ibn Baaz, 23/416, 417