It is narrated from Sayyiduna Ammar bin Yasir (may Allah be pleased with him) that he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Truly, Allah, the Blessed and Exalted, has granted me an angel from among the angels who will stand (appointed) at my grave after my death. So whenever any servant sends blessings upon me, that angel will say (in my presence): 'O Muhammad, so-and-so son of so-and-so sends blessings upon you.' The angel will call him by his own name and his father's name, and Allah, the Mighty and Majestic, in return for that one blessing, will send down ten mercies upon that person."
Imam al-Harith bin Abi Usamah recorded this hadith in his Musnad; its chain contains Abd al-Aziz bin Aban al-Umawi, who is matruk (abandoned) and da'if (weak) according to the hadith scholars. However, the basic subject of the virtues of sending blessings and of blessings being conveyed through angels is established by other sahih hadith and supporting evidence.
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / كتاب الأذكار والدعوات / 3326
Hadith Gradingمحدثین:إسناده ضعيف جدًا
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :الحديث بهذا الإِسناد ضعيف جدًا، لوجود عبد العزيز بن أبان، وفيه نُعيم بن ضَمْضَم وهو ضعيف، وفيه عمران الحِمْيري وهو لا يعرف. » «أخرجه مسند البزار برقم: 1425»