It's not in the Constitution, but it's in the other writings this country treasures. The bible.
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Thou Shalt not steal is also in the bible. Charity has no purpose in Government:
http://www.neoperspectives.com/foundingoftheunitedstates.htm
The government of the United States is a definite government, confined to specified objects. It is not like state governments, whose powers are more general. Charity is no part of the legislative duty of the government.
James Madison
I feel obliged to withhold my approval of the plan to indulge in benevolent and charitable sentiment through the appropriation of public funds. ... I find no warrant for such an appropriation in the Constitution.
Grover Cleveland
I cannot find any authority in the Constitution for public charity.
Franklin Pierce
The article, "Not Yours to Give," has been posted a nunber of times on FreeRepublic, so I'll just link the most recent, during the Congressional spending spree that resulted from last December's tsunami in the Indian Ocean.
You'll note the usual suspects.