Sa'id bin al-Musayyab narrates: I attended the funeral of Zayd bin Thabit (may Allah be pleased with him). When he was lowered into his grave, Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "O people, whoever would like to know how knowledge departs, this is how knowledge departs. By Allah, today a great deal of knowledge has gone." Sa'id bin al-Musayyab said: And the one who said that a great deal of knowledge had gone was Ibn Abbas (may Allah be pleased with them both) himself.
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4734