Musnad Ahmad 32:1810

Abu Nu'aym narrated to us, Yunus narrated to us, he said: Burayd ibn Abi Maryam narrated to me, he said: Anas ibn Malik narrated to me, he said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever sends one prayer upon me, Allah sends ten prayers upon him, and removes ten sins from him."