Deuteronomy 25:3

Forty stripes he may give him, and not exceed: lest, if he should exceed, and beat him above these with many stripes, then thy brother should seem vile unto thee.

Cross-references

2 Corinthians 11:24 explicitly says Paul received 'forty lashes minus one' — a direct reference to the limit in Dt 25:3, showing the law's application.