use ctx.Flash as per @bkcsoft comment

options/locale/locale_en-US.ini

@@ -753,6 +753,7 @@ issues.tracker = Time Tracker
 issues.start_tracking_short = Start
 issues.start_tracking = Start Time Tracking
 issues.start_tracking_history = `started working %s`
+issues.tracker_auto_close = Timer would be stopped automatically when this issue is closed
 issues.tracking_already_started = `You have already started time tracking on this <a href="%s">issue</a>!`
 issues.stop_tracking = Stop
 issues.stop_tracking_history = `stopped working %s`

routers/repo/issue_stopwatch.go

@@ -17,6 +17,13 @@ func IssueStopwatch(c *context.Context) {
 	if c.Written() {
 		return
 	}
+
+	var showSuccessMessage bool
+
+	if !models.StopwatchExists(c.User.ID, issue.ID) {
+		showSuccessMessage = true
+	}
+
 	if !c.Repo.CanUseTimetracker(issue, c.User) {
 		c.NotFound("CanUseTimetracker", nil)
 		return
@@ -27,6 +34,10 @@ func IssueStopwatch(c *context.Context) {
 		return
 	}
 
+	if showSuccessMessage {
+		c.Flash.Success(c.Tr("repo.issues.tracker_auto_close"))
+	}
+
 	url := issue.HTMLURL()
 	c.Redirect(url, http.StatusSeeOther)
 }

templates/repo/issue/view_content/comments.tmpl

@@ -1,3 +1,4 @@
+{{ template "base/alert" }}
 {{range .Issue.Comments}}
 	{{ $createdStr:= TimeSinceUnix .CreatedUnix $.Lang }}