Al-Awzaa’ee narrates that Ibn Shihaab az-Zuhree 124 AH said,

إنما يُذهب العلم نسيان ،بترك المذاكرة .

”Knowledge is lost through forgetfulness and no revision”

Taken from: Jaami’ul Bayaan al-‘ilm wa fadhlihi volume 1 page 213