diff --git a/rest_framework/validators.py b/rest_framework/validators.py
index 8006e8788..4c444cf01 100644
--- a/rest_framework/validators.py
+++ b/rest_framework/validators.py
@@ -188,10 +188,8 @@ class UniqueTogetherValidator:
                 if attrs[field_name] != getattr(serializer.instance, field_name)
             ]
 
-        condition_kwargs = {
-            serializer.fields[field_name].source: attrs[serializer.fields[field_name].source]
-            for field_name in self.condition_fields
-        }
+        condition_sources = (serializer.fields[field_name].source for field_name in self.condition_fields)
+        condition_kwargs = {source: attrs[source] for source in condition_sources}
         if checked_values and None not in checked_values and qs_exists_with_condition(queryset, self.condition, condition_kwargs):
             field_names = ', '.join(self.fields)
             message = self.message.format(field_names=field_names)