Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: We were with the Prophet (peace and blessings be upon him) when the moon split, and one piece of it went behind the mountain. Then the Prophet (peace and blessings be upon him) said: "Bear witness, bear witness."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9973