Sayyiduna Abdullah bin Amr (may Allah be pleased with him) says: "Whoever sends blessings upon the Prophet (peace and blessings be upon him) once, Allah Almighty and His angels send mercy upon that person 70 times."
Ahmad recorded it, and its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدعية / 17283