diff --git a/prompb/remote.pb.go b/prompb/remote.pb.go
index d61048134..ad91fa5b9 100644
--- a/prompb/remote.pb.go
+++ b/prompb/remote.pb.go
@@ -128,6 +128,7 @@ func (m *Query) GetMatchers() []*LabelMatcher {
 }
 
 type QueryResult struct {
+	// Samples within a time series must be ordered by time.
 	Timeseries []*TimeSeries `protobuf:"bytes,1,rep,name=timeseries" json:"timeseries,omitempty"`
 }
 
diff --git a/prompb/remote.proto b/prompb/remote.proto
index 6049d5468..172629792 100644
--- a/prompb/remote.proto
+++ b/prompb/remote.proto
@@ -38,5 +38,6 @@ message Query {
 }
 
 message QueryResult {
+  // Samples within a time series must be ordered by time.
   repeated prometheus.TimeSeries timeseries = 1;
 }