Musnad Ahmad 30:443

Rab'i narrated to us, Abd al-Rahman ibn Ishaq narrated to us, from al-Ala' ibn Abd al-Rahman, from his father, from Abu Hurayrah, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Whoever sends blessings upon me once, Allah the Almighty and Majestic writes for him ten good deeds because of it."