مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4242
Musnad of Abdullah bin Mas'ood
It was narrated from ‘Abdullah that He said concerning this verse: “The Hour has drawn near, and the moon has been cleft asunder” (al-Qamar 54:1): It was split at the time of the Messenger of Allah (ﷺ) into two halves, one half was behind the mountain and the other one half was over the mountain. And The Messenger of Allah (ﷺ) said: "O Allah, bear witness."
Musnad Ahmad 4270Its isnad is saheeh, al-Bukhari (4864) and Muslim (2800)]