PHP error unexpected '=>' (T_DOUBLE_ARROW)

I'm using the following code in Wordress to create my nav but for some reason, I keep getting the same error.

<nav>
    <?php wp_nav_menu( 'menu_id' => 'navigation', 'menu_class' => 'slimmenu'); ?>  
</nav>

error

 Parse error: syntax error, unexpected '=>' (T_DOUBLE_ARROW) 

Many thanks, P