diff --git a/sklearn/model_selection/_validation.py b/sklearn/model_selection/_validation.py
index 9e27ee28005dc..aaa25e2ab2c25 100644
--- a/sklearn/model_selection/_validation.py
+++ b/sklearn/model_selection/_validation.py
@@ -1039,6 +1039,7 @@ def permutation_test_score(estimator, X, y, groups=None, cv=None,
         Ojala and Garriga. Permutation Tests for Studying Classifier
         Performance.  The Journal of Machine Learning Research (2010)
         vol. 11
+        `[pdf] <http://www.jmlr.org/papers/volume11/ojala10a/ojala10a.pdf>`_.
 
     """
     X, y, groups = indexable(X, y, groups)