He narrated to them that the people of Mecca asked the Messenger of Allah ﷺ to show them a sign, so he showed them the moon split into two pieces, such that they saw Mount Hira between them.
أنه حدثهم أن أهل مكة سألوا رسول الله ﷺ أن يريهم آية، فأراهم القمر شقتين، حتى رأوا حراء بينهما.
Bukhari narrated it in the Signs of Prophethood (3868) and Muslim in the Characteristics of the Hypocrites (46:2802), both from the hadith of Qatada, from Anas bin Malik, who related it. Anas was four or five years old in Madinah, and it is possible that he was among those who witnessed the splitting of the moon.