Sayyiduna Ka'b bin Malik (may Allah be pleased with him) said: When the Messenger of Allah (peace and blessings be upon him) was pleased by something, his face would light up as though it were a piece of the moon, so that his pleasure would be known from it.
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16184