Majma al-Zawaid — Hadith 574

Sayyiduna Jabir (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) said: "Among the sources of piety (taqwa) is that, along with what you know, you also acquire knowledge of what you do not know. The deficiency in what you know is the scantiness of increase in it. A man does not become indifferent to knowledge except when he finds that he has gained little benefit from the knowledge he has acquired."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains Yasin al-Zayyat, who is munkar al-hadith (his narrations are rejected).