Android Api 24 can't find sources

I have this grade file:

//ext.support_library_version = '24.0.0'

    android {
        compileSdkVersion 24
        buildToolsVersion '24.0.0'

        defaultConfig {
            applicationId "---"
            minSdkVersion 21
            targetSdkVersion 24
            versionCode 1
            versionName "1.0"
        }
    ...
    }

But then when i try to open any android related class, it says i haven't got sources for api24, even tho the buildToolsVersion '24.0.0' is installed. Any idea why?