Musnad Ahmad 26:720

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Sulayman, from Ibrahim, from Abu Ma‘mar, from ‘Abdullah, that he said regarding this verse: {The Hour has drawn near, and the moon has split} [Al-Qamar: 1]: It split during the time of the Messenger of Allah (may Allah bless him and grant him peace) into two parts, or two halves—Shu‘bah is the one who is uncertain—so one half was behind the mountain, and one half was upon the mountain. The Messenger of Allah (may Allah bless him and grant him peace) said: "O Allah, bear witness."