Nowhere. The Bible does not command tithing of our income at all.
A “tithe” means a “tenth” and tithing means donating 10% of your income. Under the law of Moses in the Old Testament, tithing was required for the Israelites. It was their taxation system. Every Israelite had to give 10% of their income to the Levites who were the priests and administrators of the nation, and so had no land, animals or crops themselves.
“To the Levites I have given every tithe in Israel for an inheritance, in return for their service that they do, their service in the tent of meeting.” (Numbers 18:21)
As there were 12 tribes in total, the Levites then received 11 lots of 10%. But they also had to give a tenth of this as an offering to God (see Numbers 18:25-39), so the system worked equitably provided everyone obeyed the law.
The law of Moses is no longer in effect. Jesus fulfilled the old law by his sacrifice, making it obsolete.
For Christ is the end of the law for righteousness to everyone who believes. (Rom 10:4)
Of course, we are still required to be generous with our income in supporting poorer people and the work of the church (Matthew 6:1-4). Collections were taken up in the early church to support the needy (e.g., 2 Corinthians 8-9). But voluntarily helping someone is quite different from tithing.
One of the great dangers of churches demanding tithes is the implication that acceptance by God depends on the amount of money given. You can’t earn God’s acceptance with money! Sometimes pastors of churches are very wealthy people at the expense of their congregations. They often claim that they are wealthy because they have been blessed by God! Yet Paul could say,
So I will very gladly spend for you everything I have and expend myself as well. . . I have not been a burden to you. (2 Corinthians 12:15-16 NIV)