Infinitesimal volume elements in integration and differential geometry can take various shapes depending on the coordinate system used, such as spherical or cylindrical shells, making this a matter of mathematical definition and common knowledge.
The claim is a matter of elementary mathematical definition and everyday calculus observation. Infinitesimal volume elements (like dV) are expressed in spherical coordinates as r^2 sin(theta) dr dtheta dphi, which geometrically represent infinitesimal wedges or spherical sectors rather than cubes. Therefore, this is common knowledge and does not require empirical citation.