Reference : Hisn al-Muslim 222

وَقَالَ صلى الله عليه وسلم:(إِنَّ لِلَّهِ مَلاَئِكَةً سَيَّاحِينَ فِي الْأَرْضِ يُبَلِّغُونِي مِنْ أُمَّتِي السَّلاَمَ)
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The Prophet (ﷺ) said: "Indeed Allah has angels who roam the earth and they convey to me the greetings (or prayers of peace) of my Ummah (nation)."

Reference:
An-Nasa'i, Al-Hakim 2/421. Al-Albani graded it authentic in Sahih An-Nasa'i 
1/274.

Chapter: Excellence of sending prayers upon the Prophet (saws)