diff --git a/hgext/rebase.py b/hgext/rebase.py
--- a/hgext/rebase.py
+++ b/hgext/rebase.py
@@ -546,7 +546,9 @@ class rebaseruntime:
         date = self.date
         if date is None:
             date = ctx.date()
-        extra = {b'rebase_source': ctx.hex()}
+        extra = {}
+        if repo.ui.configbool(b'rebase', b'store-source'):
+            extra = {b'rebase_source': ctx.hex()}
         for c in self.extrafns:
             c(ctx, extra)
         destphase = max(ctx.phase(), phases.draft)
diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -2841,3 +2841,17 @@ coreconfigitem(
     b'experimental.inmemory',
     default=False,
 )
+
+# This setting controls creation of a rebase_source extra field
+# during rebase. When False, no such field is created. This is
+# useful eg for incrementally converting changesets and then
+# rebasing them onto an existing repo.
+# WARNING: this is an advanced setting reserved for people who know
+# exactly what they are doing. Misuse of this setting can easily
+# result in obsmarker cycles and a vivid headache.
+coreconfigitem(
+    b'rebase',
+    b'store-source',
+    default=True,
+    experimental=True,
+)