Musnad Abu Ya'la al-Mawsili 133:25

'Abd al-Rahman ibn Salam al-Jumahi narrated to us, Ibrahim ibn Tahman narrated to us, from Abu Ishaq, from Anas ibn Malik, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever I am mentioned in his presence, let him send blessings upon me, for whoever sends blessings upon me once, Allah will send blessings upon him ten times."