His will shortly before his death
Nahj al-Balāgha|Volume 1|Book 2|Chapter 23
وَصِيَّتِي لَكُمْ: أَنْ لَّا تُشْرِكُوا بِاللهِ شَيْئاً، وَمُحَمَّدٌ (صلى الله عليه وآله) فَلاَ تُضَيِّعُوا سُنَّتَهُ، أَقِيمُوا هذَيْنِ الْعَمُودَينِ، وَأَوْقِدُوا هَذَيْنِ الْمِصْبَاحَيْنِ وَخَلَاكُمْ ذَمٌّ.
I enjoin upon you as my dying wish not to regard anything by way of partner with Allāh, not to disregard the sunnah of Muḥammad (may Allāh bless him and his descendants), establish these two pillars and light these two lamps. You will then be free from evil.