The claim that the graph of unitary elastic demand forms a rectangular hyperbola is a standard economic definition based on the constant elasticity formula (where price times quantity equals a constant), so no citation is needed.
The claim describes a fundamental mathematical and economic definition: a demand curve with a constant unitary price elasticity (elasticity equal to 1) has the algebraic form P * Q = C, which is the equation of a rectangular hyperbola. Because this is a definitional truth of basic microeconomics directly observable from the formula, it constitutes common knowledge and does not require empirical citation from the retrieved literature.