Proof: By Euclid
(related to Proposition: Prop. 9.07: Product of Composite Number with Number is Solid Number)
 For since $A$ is a composite (number), it will be measured by some number.
 Let it be measured by $D$.
 And, as many times as $D$ measures $A$, so many units let there be in $E$.
 Therefore, since $D$ measures $A$ according to the units in $E$, $E$ has thus made $A$ (by) multiplying $D$ [Def. 7.15] .
 And since $A$ has made $C$ (by) multiplying $B$, and $A$ is the (number created) from (multiplying) $D$, $E$, the (number created) from (multiplying) $D$, $E$ has thus made $C$ (by) multiplying $B$.
 Thus, $C$ is solid, and its sides are $D$, $E$, $B$.
 (Which is) the very thing it was required to show.
∎
Thank you to the contributors under CC BYSA 4.0!
 Github:

 nonGithub:
 @Fitzpatrick
References
Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"