Sahih Bukhari Volume 4, Book 56, Hadith Number 756.

Narated By ‘Abdullah bin Ka’b : I heard Ka’b bin Malik talking after his failure to join (the Ghazwa of) Tabuk. He said, “When I greeted Allah’s Apostle whose face was glittering with happiness, for whenever Allah’s Apostle was happy, his face used to glitter, as if it was a piece of the moon, and we used to recognize it (i.e. his happiness) from his face.”