Four things, one of them being kept alive too long
He asked not to be returned to it.
أَعُوذُ بِكَ مِنَ الْجُبْنِ، وَأَعُوذُ بِكَ أَنْ أُرَدَّ إِلَى أَرْذَلِ الْعُمُرِ، وَأَعُوذُ بِكَ مِنْ فِتْنَةِ الدُّنْيَا، وَأَعُوذُ بِكَ مِنْ عَذَابِ الْقَبْرِ
Aʿūdhu bika mina-l-jubn, wa aʿūdhu bika an uradda ilā ardhali-l-ʿumur, wa aʿūdhu bika min fitnati-d-dunyā, wa aʿūdhu bika min ʿadhābi-l-qabr
“I seek refuge in You from cowardice, and from being returned to the most feeble age, and from the trial of this world, and from the punishment of the grave.”
Where this comes from
Ardhal al-ʿumur is the stage where a person who knew things stops knowing them. It is asked about in the Qurʾān in the same words. Not death. The part before it.
The full narration
Narrated `Amr bin Maimun Al-Audi:Sa`d used to teach his sons the following words as a teacher teaches his students the skill of writing and used to say that Allah's Messenger (ﷺ) used to seek Refuge with Allah from them (i.e. the evils) at the end of every prayer. The words are: 'O Allah! I seek refuge with You from cowardice, and seek refuge with You from being brought back to a bad stage of old life and seek refuge with You from the afflictions of the world, and seek refuge with You from the punishments in the grave
Source
Ṣaḥīḥ al-Bukhārī · 2822
- Agreed upon as authentic (al-Bukhārī / Muslim)