diff --git a/federationapi/routing/join.go b/federationapi/routing/join.go
index c0bec0449..ada89e4a6 100644
--- a/federationapi/routing/join.go
+++ b/federationapi/routing/join.go
@@ -275,6 +275,7 @@ func SendJoin(
 		}
 	}
 
+	// https://matrix.org/docs/spec/server_server/latest#put-matrix-federation-v1-send-join-roomid-eventid
 	return util.JSONResponse{
 		Code: http.StatusOK,
 		JSON: gomatrixserverlib.RespSendJoin{