Hadith: During the Prophet's lifetime, the moon split into two halves. The mountain covered one half, and the other half was above the mountain. The Messenger of Allah (may Allah’s peace and blessings be upon him) said: O Allah, bear witness to this

عن عبد الله بن مسعود رضي الله عنه مرفوعاً: انشقَّ القمر على عهد رسول الله صلى الله عليه وسلم فِلْقَتين، فستر الجبل فِلْقَة، وكانت فِلْقَة فوق الجبل، فقال رسول الله صلى الله عليه وسلم : «اللهمَّ اشهَدْ».
[صحيح] - [متفق عليه]

‘Abdullāh ibn Mas‘ūd (may Allah be pleased with him) reported that during the Prophet's lifetime, the moon split into two halves. The mountain covered one half, and the other half was above the mountain. The Messenger of Allah (may Allah’s peace and blessings be upon him) said: "O Allah, bear witness to this!"
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

During the time of the Messenger of Allah (may Allah’s peace and blessings be upon him), the moon split into two separate pieces each piece in a place. Thereupon, the Messenger of Allah (may Allah’s peace and blessings be upon him) said: "O Allah, bear witness to this!" i.e. O Allah, bear witness to the fact that I showed them a sign and a miracle that proves my prophet-hood.