Musannaf Ibn Abi Shayba 27:153

Waki’ narrated to us, from Shu’bah, from ‘Asim ibn ‘Abdullah, from ‘Abdullah ibn ‘Amir ibn Rabi’ah, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever sends blessings upon me, the angels do not cease sending blessings upon him as long as he sends blessings upon me. So let the servant do little of that or much."