Musnad Ahmad 30:3157

'Abd al-Rahman narrated to us, from Zuhayr, and Abu 'Amir narrated to us, Zuhayr narrated to us, from al-'Ala', from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever sends blessings upon me once, Allah sends blessings upon him ten times."