When in Samus' ship, go to the radio and press the following symbols:

Going from left to right, press the eighth symbol, the third symbol, the fifth symbol and then the fourth symbol.

You will then hear a message from Mr. Miyamoto!

A rough translation:

Ah, can you hear me? This is Miyamoto.
All of you playing Metroid, can you hear me?
It's fun, isn't it?
I've also worked on many [Metroid] games, but this one is the best yet, don't you think?
Definitely play it all the way to the end.
See you!