The Imams عليه السلام know when they will die and they die voluntarily
Al-Kāfi|Volume 1|Book 4|Chapter 47
6ـ مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنِ الْوَشَّاءِ عَنْ مُسَافِرٍ أَنَّ أَبَا الْحَسَنِ الرِّضَا (عَلَيْهِ السَّلام) قَالَ لَهُ يَا مُسَافِرُ هَذَا الْقَنَاةُ فِيهَا حِيتَانٌ قَالَ نَعَمْ جُعِلْتُ فِدَاكَ فَقَالَ إِنِّي رَأَيْتُ رَسُولَ الله (صَلَّى اللهُ عَلَيْهِ وَآلِه) الْبَارِحَةَ وَهُوَ يَقُولُ يَا عَلِيُّ مَا عِنْدَنَا خَيْرٌ لَكَ.
6. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad from al-Washsha’ from Musafir that abu al-Hassan al-Rida عليه السلام said to him the following. “O Musafir, in this water passage there is fish.” The narrator has said that he said, “Yes, “May Allah take my soul in service for your cause, there is fish there.” The Imam عليه السلام then said, “I saw the Messenger of Allah last night and was saying, “O Ali, what is with us is better for you.” (The mention of fish is to show equality of degree of certain in his knowledge from observation and his knowledge from his dream.)
- Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (3/127)
- Ṣaḥīḥ al-Kāfī