Musnad Abu Ya'la al-Mawsili 238:4

Ishaq ibn Abi Isra'il narrated to us, Nadr ibn Shumayl informed us, Shu'bah informed us, 'Asim ibn 'Ubayd Allah narrated to us, he said: I heard 'Abd Allah ibn 'Amir ibn Rabi'ah mentioning from his father that he heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "There is no servant who sends blessings upon me with a prayer except that the angels send blessings upon him for as long as he sends blessings upon me. So let the servant do little of that or let him do much."