diff --git a/UPGRADE.md b/UPGRADE.md
index f7c1165bdff..d004400e53d 100644
--- a/UPGRADE.md
+++ b/UPGRADE.md
@@ -1,3 +1,10 @@
+# Upgrade to 3.1
+
+## Deprecated `Connection::$_expr`
+
+The usage of `Connection::$_expr` by extending classes is deprecated. Use `Connection::getExpressionBuilder()` instead.
+Do not rely on the same builder instance being returned by each invocation.
+
 # Upgrade to 3.0
 
 ## BC BREAK: leading colon in named parameter names not supported
diff --git a/src/Connection.php b/src/Connection.php
index 4006d60bdcc..9d2181d0235 100644
--- a/src/Connection.php
+++ b/src/Connection.php
@@ -68,7 +68,11 @@ class Connection
     /** @var EventManager */
     protected $_eventManager;
 
-    /** @var ExpressionBuilder */
+    /**
+     * @deprecated Use {@link getExpressionBuilder()} instead.
+     *
+     * @var ExpressionBuilder
+     */
     protected $_expr;
 
     /**