Musnad Abu Ya'la al-Mawsili 124:1

Abu Sa’id al-Ashajj and others narrated to us, they said: Abu Khalid narrated to us, from ‘Isa ibn Maysarah, from Abu al-Zinad, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Prayer is the light of the believer."