D&C 119:3-4 reports the beginning of tithing in the Church in July 1838.
Section 119:3-4, Doctrine and Covenants, The Church of Jesus Christ of Latter-day Saints, accessed September 25, 2023
3 And this shall be the beginning of the tithing of my people.
4 And after that, those who have thus been tithed shall pay one-tenth of all their interest annually; and this shall be a standing law unto them forever, for my holy priesthood, saith the Lord.