Nahj al-Balāgha
Hadith 1923٣٦٥. وَقَالَ (عليه السلام): الْفِكْرُ مرْآةٌ صَافِيَةٌ، وَالِاعْتِبَارُ مُنْذِرٌ نَاصِحٌ، وَكَفَى أَدَباً لِنَفْسِكَ تَجَنُّبُكَ مَا كَرِهْتَهُ لِغَيْرِكَ.
365. Amīr al-mu’minīn, peace be upon him, said: Imagination is a clear mirror, and the taking of lessons (from things around) provides warning and counsel. It is enough for improving yourself that you should avoid what you consider bad in others.