D.C. Code § 21-150
Every fact material to determine the propriety of a sale or exchange shall be clearly proved, in a proceeding brought pursuant to section 21-148 , by disinterested witnesses, whose testimony shall be taken in writing in the presence of the guardian ad litem or upon interrogatories agreed upon by him.
Sept. 14, 1965, 79 Stat. 743, Pub. L. 89-183, § 1
1973 Ed., § 21-150.
1981 Ed., § 21-150.