Musannaf Ibn Abi Shayba 33:734

Sufyan ibn ‘Uyaynah narrated to us, from Ibn ‘Ajlan, from ‘Awn ibn ‘Abd Allah, who said: Indeed, part of the perfection of piety (taqwa) is that you seek, regarding what you have learned of it, the knowledge of what you have not yet learned. And know that among what you have learned is the abandonment of seeking increase in it. And a man is only driven to abandon seeking increase in what he has already learned by the lack of benefit he derives from what he has already learned.