diff --git a/tests/qgis_interface.py b/tests/qgis_interface.py
index 5ffc59f..bed572e 100644
--- a/tests/qgis_interface.py
+++ b/tests/qgis_interface.py
@@ -12,6 +12,7 @@
      Copyright (c) 2011 German Carrillo, geotux_tuxman@linuxmail.org
      Copyright (c) 2014 Tim Sutton, tim@linfiniti.com
 
+     reference: https://github.com/felt/qgis-plugin/blob/main/felt/test/qgis_interface.py
 """
 
 __author__ = "tim@linfiniti.com"
diff --git a/tests/test_qgis_environment.py b/tests/test_qgis_environment.py
index f32bae8..91c4d86 100644
--- a/tests/test_qgis_environment.py
+++ b/tests/test_qgis_environment.py
@@ -1,4 +1,5 @@
 """
+reference: https://github.com/felt/qgis-plugin/blob/main/felt/test/test_qgis_environment.py
 Tests for QGIS functionality.
 """
 
diff --git a/tests/utilities.py b/tests/utilities.py
index 22446c6..3db60f0 100644
--- a/tests/utilities.py
+++ b/tests/utilities.py
@@ -1,4 +1,5 @@
 """
+reference: https://github.com/felt/qgis-plugin/blob/main/felt/test/utilities.py
 Common functionality used by regression tests.
 """