25 U.S.C. § 158: Investment of proceeds of lands

The Secretary of the Interior shall invest in a manner which shall be in his judgment most safe and beneficial for the fund, all moneys that may be received under treaties containing stipulations for the payment to the Indians, annually, of interest upon the proceeds of the lands ceded by them; and he shall make no investment of such moneys, or of any portion, at a lower rate of interest than 5 per centum per annum.