Sahih Muslim
The Book Giving Description of the Day of Judgement, Paradise and Hell (Kitab Sifat Al-Qiyamah wal Janna wan-Nar)
Book Number: 039 / Hadith Number: 6726
'Abdullah b. Mas'ud reported that the moon was split up in two parts during the lifetime of Allah's Messenger (may peace be upon him). The mountain covered one of its parts and one part of it was above the mountain and Allah's Messenger (may peace be upon him) said: Bear witness to this.