It is narrated from Sayyiduna Ibn Amr, may Allah be pleased with him, that whoever sends blessings upon the Noble Prophet, peace and blessings be upon him, once, Allah and His angels send mercy upon him seventy times. Now it is up to the servant whether he increases or decreases the sending of blessings.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 6605
Hadith Gradingحکم دارالسلام:إسناده ضعيف، ابن لهيعة سييء الحفظ، وعبد الرحمن بن مريح مجهول