diff --git a/src/ImageSharp/Formats/Heif/Av1/Readme.md b/src/ImageSharp/Formats/Heif/Av1/Readme.md
index 056d351cb..6abfa731c 100644
--- a/src/ImageSharp/Formats/Heif/Av1/Readme.md
+++ b/src/ImageSharp/Formats/Heif/Av1/Readme.md
@@ -46,3 +46,7 @@ A block is the smallest are of the image which has the same transformation param
 ## ModeInfo
 
 The smallest unit in the frame. It determines the parameters for an area of 4 by 4 pixels.
+
+# References
+
+[AV1 specification](https://aomediacodec.github.io/av1-spec/av1-spec.pdf)