Authentic Supplication Number 62.

The Prophet of Allah  said: “If you hear the mu’ath-thin then say as he says, then make salat (79) upon me, for whoever makes salat upon me once; Allah will make salat upon him ten times, then ask Allah for the Waseelata for me, for it is a station in Paradise which is not befitting for but one of Allah’s bondservants, and I hope to be that bondservant, so whoever asks for the Waseelata for me, my intercession would become lawful for him”.

(79) For example to say: Allahumma Salli `ala muhammmad.

Reported by Muslim, and is #55 in The Authentic of Good Sayings.