diff --git a/doc/04-schema.md b/doc/04-schema.md
index 74c7510de..f08290ace 100644
--- a/doc/04-schema.md
+++ b/doc/04-schema.md
@@ -581,6 +581,8 @@ The following options are supported:
   higher if you have a slow connection or huge vendors.
 * **use-include-path:** Defaults to `false`. If true, the Composer autoloader
   will also look for classes in the PHP include path.
+* **prefer-source:** Defaults to `false`. If true, Composer will always prefer
+  source installs.
 * **github-protocols:** Defaults to `["git", "https", "http"]`. A list of
   protocols to use for github.com clones, in priority order. Use this if you are
   behind a proxy or have somehow bad performances with the git protocol.
diff --git a/res/composer-schema.json b/res/composer-schema.json
index e8c0474e7..d6a658e5d 100644
--- a/res/composer-schema.json
+++ b/res/composer-schema.json
@@ -116,6 +116,10 @@
                     "type": "boolean",
                     "description": "If true, the Composer autoloader will also look for classes in the PHP include path."
                 },
+                "prefer-source": {
+                    "type": "boolean",
+                    "description": "If true, Composer will always prefer source installs."
+                },
                 "notify-on-install": {
                     "type": "boolean",
                     "description": "Composer allows repositories to define a notification URL, so that they get notified whenever a package from that repository is installed. This option allows you to disable that behaviour, defaults to true."