Android : How to download sources from specific dependency gradle

I am trying to know how to get gradle to download the source of a given dependency. All that I've searched has not work. I've seen the property DownloadSources. But I do not know where to write it in my Android gradle file.

I've tried to write it in different locations, inside dependencies {}, android { }, defaultConfig {}. But no luck. In a regular gradle file like this, where have I to use downloadsources?:

Project file

sub-projects/modules.
buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        //..
    }
}

allprojects {
    repositories {
        // ..
    }
} 

Module file

apply plugin: 'com.android.application'

android {
    compileSdkVersion 22
    buildToolsVersion "22.0.0"

    defaultConfig {
        // ...
    }
    buildTypes {
        release {
            //....
        }
    }
}

dependencies {
    // ...
    compile 'it.neokree:MaterialNavigationDrawer:latest.release'
}

Best regards and sorry if its a dummy question, but I can not find a solution.