diff --git a/docs/reference/docs/reindex.asciidoc b/docs/reference/docs/reindex.asciidoc
index 314f63c7ada..bf8a2057b39 100644
--- a/docs/reference/docs/reindex.asciidoc
+++ b/docs/reference/docs/reindex.asciidoc
@@ -1,6 +1,9 @@
 [[docs-reindex]]
 == Reindex API
 
+IMPORTANT: Reindex requires <<mapping-source-field,`_source`>> to be enabled for
+all documents in the source index.
+
 IMPORTANT: Reindex does not attempt to set up the destination index.  It does
 not copy the settings of the source index.  You should set up the destination
 index prior to running a `_reindex` action, including setting up mappings, shard