Add original PR reference
diff --git a/ChangeLog b/ChangeLog
index 25f52c8..8a3f0af 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -4,7 +4,7 @@
 
 Changes
    * Optimize unnecessary zeroing in mbedtls_mpi_copy. Based on a contribution
-     by Alexey Skalozub.
+     by Alexey Skalozub in #405.
 
 = mbed TLS 2.7.0 branch released 2018-02-03