Amos 4:5
3 Each of you will go straight through the gaps in the walls; you will be thrown out toward Harmon.” The Lord is speaking. 4 “Go to Bethel and rebel! At Gilgal rebel some more! Bring your sacrifices in the morning, your tithes on the third day! 5 Burn a thank offering of bread made with yeast! Make a public display of your voluntary offerings! For you love to do this, you Israelites.” The Sovereign Lord is speaking. 6 “But surely I gave you no food to eat in all your cities; you lacked food everywhere you lived. Still you did not come back to me.”The Lord is speaking. 7 “I withheld rain from you three months before the harvest. I gave rain to one city, but not to another. One field would get rain, but the field that received no rain dried up.
Matthew 6:2
1 “Be careful not to display your righteousness merely to be seen by people. Otherwise you have no reward with your Father in heaven. 2 Thus whenever you do charitable giving, do not blow a trumpet before you, as the hypocrites do in synagogues and on streets so that people will praise them. I tell you the truth, they have their reward! 3 But when you do your giving, do not let your left hand know what your right hand is doing, 4 so that your gift may be in secret. And your Father, who sees in secret, will reward you.
Notes and References
"... No particular biblical law is cited at the outset of the section on free-will offerings. In fact, references to these offerings are infrequent in scripture. The term is used for the contribution incumbent on all Israelites, both for the construction of the wilderness sanctuary (Exodus 35:29; 36:3) and for the rebuilding of the Temple after the exile (Ezra 1:4; 8:28). Legal discussion of the nature of the offering is limited to the stipulations that the offering may be eaten on the day of sacrifice and the day after (Leviticus 7:15-19), that it must be unblemished (Leviticus 22:17-25), that it be offered in the one sanctuary (in Jerusalem, Deuteronomy 12:6-7), that its amount be proportionate to one's prosperity (Deuteronomy 16:9-12; compare Sirach 35:7-9), and that one must make good on free-will vows. (compare Deuteronomy 23:22-24; Proverbs 20:25; Ecclesiastes 5:3-6. One text from Amos 4:5 indicates that people often proclaimed their free-will offerings publicly; compare Matt 6:1-6. Free-will offerings are also mentioned in lists of festival sacrifices: Leviticus 23:38; Numbers 29:39; Ezra 3:5, and procedures are given for the free-will offering of a prince: Ezekiel 46:12) There is, however, no biblical statute prohibiting the dedication of either the food of one's mouth or something taken by force, although there are several prophetic and sapiential texts that criticize sacrifices acquired by robbery ..."
Murphy, Catherine M. Wealth in the Dead Sea Scrolls and in the Qumran Community (p. 62) Brill, 2002