Musnad Ahmad 1227:11

Yahya ibn Adam narrated to us, he said: Ibn al-Mubarak narrated to us, from Ma’mar and Yunus, from al-Zuhri, from ‘Abd al-Rahman ibn ‘Abd Allah ibn Ka’b ibn Malik, from Ka’b ibn Malik, who said: "When the Messenger of Allah, peace and blessings of Allah be upon him, was happy, his face would illuminate, as if his face were a piece of the moon." And we used to recognize that in him.