I'm using the slack-notifier gem to send notifications to my slack channel.
notifier = Slack::Notifier.new "https://hooks.slack.com/services/ABC1234567890"
notifier.ping "Text <@user1>"
(Moving my comment to an answer.)
I'd suggest this:
notifier.ping "Text @user1", parse: "full"
The "full" parse mode means you'll get automatic linking of @username, #channelname, etc., just like you get when typing into the Slack website/clients.