diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index dba347f..2775def 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -32,7 +32,7 @@ $ git checkout -b bug-or-feature-description
 And install the dependencies:
 
 ```bash
-$ composer install
+$ composer install --prefer-source
 ```
 
 Write your code and add tests. Then run the tests: