Mishkat al-Masabih 2 Knowledge كتاب العلم 62

وَعَنْ سُفْيَانَ أَنَّ عُمَرَ بْنَ الْخَطَّابِ رَضِيَ اللَّهُ عَنْهُ قَالَ لِكَعْبٍ: مَنْ أَرْبَابُ الْعِلْمِ؟ قَالَ: الَّذِي يَعْمَلُونَ بِمَا يَعْلَمُونَ. قَالَ: فَمَا أَخْرَجَ الْعِلْمَ مِنْ قُلُوبِ الْعُلَمَاءِ؟ قَالَ الطَّمَعُ. رَوَاهُ الدَّارِمِيُّ

Sufyan said that ‘Umar b. al-Khattab asked Ka‘b who were the lords of knowledge, and he replied that they are those who act according to what they know. He then asked what it is that takes knowledge from the hearts of the learned and was told that it is covetousness. Darimi transmitted it.