Ecclesiastes 12:12

And further, by these, my son, be admonished: of making many books there is no end; and much study is a weariness of the flesh.

Cross-reference

In Ecclesiastes 1:18, the same theme of study causing sorrow reinforces the weariness from much study here.

Ecclesiastes 1:13 describes the quest for wisdom as a grievous task, directly underlying the futility of endless study here.

Ecclesiastes 7:16 warns against being overwise, connecting to the self-destructive weariness from much study here.