diff --git a/skimage/restoration/_nl_means_denoising.pyx b/skimage/restoration/_nl_means_denoising.pyx
index 089914fc..62ba2e96 100644
--- a/skimage/restoration/_nl_means_denoising.pyx
+++ b/skimage/restoration/_nl_means_denoising.pyx
@@ -359,6 +359,11 @@ cdef inline float _integral_to_distance_2d(IMGDTYPE [:, ::] integral,
     """
     References
     ----------
+    J. Darbon, A. Cunha, T.F. Chan, S. Osher, and G.J. Jensen, Fast
+    nonlocal filtering applied to electron cryomicroscopy, in 5th IEEE
+    International Symposium on Biomedical Imaging: From Nano to Macro,
+    2008, pp. 1331–1334.
+
     Jacques Froment. Parameter-Free Fast Pixelwise Non-Local Means
     Denoising. Image Processing On Line, 2014, vol. 4, p. 300-326.
 
@@ -381,6 +386,11 @@ cdef inline float _integral_to_distance_3d(IMGDTYPE [:, :, ::] integral,
     """
     References
     ----------
+    J. Darbon, A. Cunha, T.F. Chan, S. Osher, and G.J. Jensen, Fast
+    nonlocal filtering applied to electron cryomicroscopy, in 5th IEEE
+    International Symposium on Biomedical Imaging: From Nano to Macro,
+    2008, pp. 1331–1334.
+
     Jacques Froment. Parameter-Free Fast Pixelwise Non-Local Means
     Denoising. Image Processing On Line, 2014, vol. 4, p. 300-326.
 
@@ -523,6 +533,16 @@ def _fast_nl_means_denoising_2d(image, int s=7, int d=13, float h=0.1):
     -------
     result : ndarray
         Denoised image, of same shape as input image.
+
+    References
+    ----------
+    J. Darbon, A. Cunha, T.F. Chan, S. Osher, and G.J. Jensen, Fast
+    nonlocal filtering applied to electron cryomicroscopy, in 5th IEEE
+    International Symposium on Biomedical Imaging: From Nano to Macro,
+    2008, pp. 1331–1334.
+
+    Jacques Froment. Parameter-Free Fast Pixelwise Non-Local Means
+    Denoising. Image Processing On Line, 2014, vol. 4, p. 300-326.
     """
     if s % 2 == 0:
         s += 1  # odd value for symmetric patch
@@ -623,6 +643,16 @@ def _fast_nl_means_denoising_3d(image, int s=5, int d=7, float h=0.1):
     -------
     result : ndarray
         Denoised image, of same shape as input image.
+
+    References
+    ----------
+    J. Darbon, A. Cunha, T.F. Chan, S. Osher, and G.J. Jensen, Fast
+    nonlocal filtering applied to electron cryomicroscopy, in 5th IEEE
+    International Symposium on Biomedical Imaging: From Nano to Macro,
+    2008, pp. 1331–1334.
+
+    Jacques Froment. Parameter-Free Fast Pixelwise Non-Local Means
+    Denoising. Image Processing On Line, 2014, vol. 4, p. 300-326.
     """
     if s % 2 == 0:
         s += 1  # odd value for symmetric patch