Here's why:
* The saying means: "Rahiman (a wise person) says, without water, everything is empty."
* The alankar: The saying uses the word "sab" (everything) implicitly comparing it to a vessel without water, which is empty and useless.
* Implied comparison: It implies a comparison between "everything" and "a vessel without water".
Therefore, the alankar in the Hindi saying "Rahiman pani rakhia bin sab sun" is Upama, where everything is likened to a vessel without water.