Musnad Ahmad 30:444

Abu Kamil narrated to us, Hammad narrated to us, from Suhayl ibn Abi Salih, 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 for it."