It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) was pleased, his (peace and blessings be upon him) face would shine as though it were a piece of the moon."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4739
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[ق] عن كعب بن مالك. مختصر مسلم ١٩١٨.