Musnad Ahmad 30:1761

Sulayman ibn Dawud narrated to us, Isma'il—meaning Ibn Ja'far—narrated to us, Al-'Ala' informed me, 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."