Answer:
Specifically Proverbs 13:24
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see also Proverbs 19:18, 22:15, 29:15, 23:13-14
For anyone wishing an answer that doesn't require them to take out the family bible, I will point out that the rod in question is the one that is used to beat children who misbehave. It was believed, by those who make that statement, that if children are not violently punished, they will become bad children, who cannot be controlled and will be horribly inconsiderate.
This verse is from Proverbs 13:24