diff --git a/src/muebtransmitter_p.h b/src/muebtransmitter_p.h
index 963c54c515d05c246b7bb3632694bbc4c0bddfb2..8b5d05085acde2968fc1331b5defb56b0ed4f772 100644
--- a/src/muebtransmitter_p.h
+++ b/src/muebtransmitter_p.h
@@ -31,6 +31,10 @@ class MuebTransmitterPrivate {
   MuebTransmitter* q_ptr;
 
   // FIXME Remove in Qt 6
+  /* Reference:
+   * http://threadlocalmutex.com/?p=48
+   * http://threadlocalmutex.com/?page_id=60
+   */
   std::function<uchar(const uchar&)> reduceColor =
       [this](const uchar& color) -> uchar {
     if (configuration_.color_depth() == 3) {