مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3732
Musnad of Abdullah bin Mas'ood
It was narrated that Ibn Mas'ood (رضي الله عنه) said: The Messenger of Allah taught me "And We have indeed made the Qur'an easy to understand and remember, then is there any one who will remember (or receive admonition) (fahal min muddakir)?" (al-Qamar 54:17). A man said: O Abu ‘Abdur-Rahman, is it muddakir or mudhdhakkir? He said: The Messenger of Allah (ﷺ) taught me (to say)"muddakir."
Musnad Ahmad 3755Its isnad is saheeh al-Bukhari (3345) and Muslim (823)]