diff --git a/exo/08/15.md b/exo/08/15.md
index bae6b736d5..2c205b8e87 100644
--- a/exo/08/15.md
+++ b/exo/08/15.md
@@ -1,6 +1,6 @@
 # he hardened his heart
 
-"Pharaoh hardened his heart." 
+"Pharaoh hardened his heart." (see Exodus 4:21)
 
 # just as Yahweh had said that he would do