diff --git a/docs/source/usersguide/input.rst b/docs/source/usersguide/input.rst
index 362197439d..6aac106130 100644
--- a/docs/source/usersguide/input.rst
+++ b/docs/source/usersguide/input.rst
@@ -264,7 +264,9 @@ attributes/sub-elements:
 
   :file:
     If this attribute is given, it indicates that the source is to be read from
-    a binary source file whose path is given by the value of this element
+    a binary source file whose path is given by the value of this element. Note,
+    the number of source sites needs to be the same as the number of particles
+    simulated in a fission source generation.
 
     *Default*: None