That doesn't sound correct. Perhaps you're mixing up the rules for tithing cattle[0] in Deuteronomy 15 with the rules for approaching Mount Sinai[1] in Exodus 19.

[0] https://www.biblegateway.com/passage/?search=Deuteronomy+15%...

[1] https://www.biblegateway.com/passage/?search=Exodus+19%3A12-...