Charity
Matthew 6:19 Do not store up for you treasure upon the earth, where moth and corrosion obliterate and where thieves dig through and steal. 20 But store up for you treasures in heaven, where neither moth nor corrosion obliterate and where thieves do not dig through nor steal. 21 For where your treasure is, there also shall be your heart.
Alms-giving in the ancient world was with all certainty seen as a way of asking forgiveness from God for one's sins. However the alms-giving had to be without fanfare. If alms-giving was accompanied with fanfare, it was for the benefit of the giver, and not truly for the recipient, and therefore God will not reward it (Matthew 6:1-3)
Witnesses
Proverbs 21:13-14: “13 Whoso stoppeth his ears at the cry of the poor, he also shall cry himself, but shall not be heard. 14 A gift in secret pacifieth anger: and a reward in the bosom strong wrath.”
Apocryphal
Sirach, chapter 29: “10 Lose thy money for thy brother and thy friend, and let it not rust under a stone to be lost. 11 Lay up thy treasure according to the commandments of the most High, and it shall bring thee more profit than gold. 12 Shut up alms in thy storehouses: and it shall deliver thee from all affliction. 13 It shall fight for thee against thine enemies better than a mighty shield and strong spear.”
Septuagint, Sirach 40:24: Brethren and help are against time of trouble: but alms shall deliver more than them both.
Septuagint, Tobit 4:5-11: 5 My son, be mindful of the Lord our God all thy days, and let not thy will be set to sin, or to transgress his commandments: do uprightly all thy life long, and follow not the ways of unrighteousness. 6 For if thou deal truly, thy doings shall prosperously succeed to thee, and to all them that live justly. 7 Give alms of thy substance; and when thou givest alms, let not thine eye be envious, neither turn thy face from any poor, and the face of God shall not be turned away from thee. 8 If thou hast abundance give alms accordingly: if thou have but a little, be not afraid to give according to that little: 9 For thou layest up a good treasure for thyself against the day of necessity. 10 Because that alms do deliver from death, and suffereth not to come into darkness. 11 For alms is a good gift unto all that give it in the sight of the most High.
If we give to our brethren when we have, we see hope that we ourselves shall receive when we are in need.
Septuagint, Tobit 4:16-17: 16 Give of thy bread to the hungry, and of thy garments to them that are naked; and according to thine abundance give alms: and let not thine eye be envious, when thou givest alms. 17 Pour out thy bread on the burial of the just, but give nothing to the wicked. [The primary difference between Christianity and Communism or compulsory Socialism.]
Eye of Sincerity
Matthew 6:22 The lamp of the body is the eye. Therefore if your eye should be sincere, your whole body shall be bright. 23 But if your eye should be evil, your whole body shall be dark. Then if the light in you is darkness, how great is that darkness!
From Deuteronomy 15:9: “Beware that there be not a thought in thy wicked heart, saying, The seventh year, the year of release, is at hand; and thine eye be evil against thy poor brother, and thou givest him nought; and he cry unto the LORD against thee, and it be sin unto thee.” From the Wisdom of Sirach, 14:10: “A wicked eye envieth his bread, and he is a niggard at his table.”
So we see that here Christ is telling us to be fair with our brother to justly give him what he is due, to share our blessings with our brethren, and to do so with sincerity.