Abdullah ibn Umar (RA) is the son of Umar ibn Khattab (RA). He was born about ten or eleven years before the migration. At the time of the event of the splitting of the moon, he was approximately five or six years old. The summary of the narrations transmitted from him in Tirmidhi and Mustadrak is as follows:
According to the narration of Tirmidhi, the moon was split during the time of the Prophet Muhammad (PBUH). He said: Be witnesses, you people.
In Al-Hakim’s Al-Mustadrak ala al-Sahihayn, there is an addition that one piece of the moon remained visible while the other went behind the mountain.
The two narrations are as follows:
حدثنا محمود بن غيلان، حدثنا ابو داؤد، عن شعبة، عن الاعمش، عن مجاهد، عن ابن عمر، قال: انفلق القمر على عهد رسول الله صلى الله عليه وسلم، فقال رسول الله صلى الله عليه وسلم: اشهدوا.
Muhammad bin Ghaylan narrated to us, he said that Abu Dawood narrated to us, from Shu‘bah, from A‘amash, and from Mujahid who narrated that ‘Abdullah bin Umar (RA) said that during the time of the Messenger of Allah (PBUH) the moon was split into two parts. The Messenger of Allah (PBUH) said: ‘Bear witness to this.’ (Tirmidhi No. 2182)
فحدثناه أبو العباس محمد بن يعقوب، ثنا إبراهيم بن مرزوق البصري بمصر، ثنا أبو داؤد الطيالسي، ثنا شعبة، عنِ الأعمشِ، عن مجاهد، عن عبد الله بن عمرو رضي اللّٰہ عنه، فِي قوله عز وجل: اقْتَرَبَتِ السَّاعَةُ وَانْشَقَّ الْقَمَرُ. قال: كان ذلك على عهد النبي صلى الله عليه وسلم، انشق القمر فلقتين فلقة من دون الجبل، وفلقة خلف الجبل. فقال النبي صلى الله عليه وسلم: اللهم اشهد.
Abu Abbas Muhammad bin Yaqub narrated to us, he said that Ibrahim bin Marzuq Basri narrated to us in Egypt, he said that Abu Dawood Tayalisi narrated to us, he said that Shu‘bah narrated to me, from A‘mash, from Mujahid, who narrated from ‘Abdullah bin Umar (RA) that while explaining the initial verse of Surah Al-Qamar, he stated that in the time of the Messenger of Allah (PBUH), the moon split into two parts. One part was in front of the mountain and the other was behind it. Upon this, the Messenger of Allah (PBUH) said: ‘O Lord, I bear witness.’ (Al-Mustadrak li’l-Hakim, No. 3759)