Instructions for Hasan, when returning from Siffin
Nahj al-Balāgha|Volume 1|Book 2|Chapter 31
وَتَفَهَّمْ وَصِيَّتِي، وَلَا تَذْهَبَنَّ عَنْكَ صَفْحاً، فَإِنَّ خَيْرَ الْقَوْلِ مَا نَفَعَ. وَاعْلَمْ أَنَّهُ لَا خَيْرَ فِي عِلْمٍ لَا يَنْفَعُ، وَلَا يُنْتَفَعُ بِعِلْمٍ لَا يَحِقُّ تَعَلُّمُهُ.
Understand my advice an do not turn away from it, because the best saying is that which benefits. Know that there is no good in that knowledge which does not benefit, and if knowledge is not made use of then its acquisition is not justified.