How do i report a hacker on Team Fortress 2 (PC).?

I was playing a game on dustbowl and I was firing at the enemy I was a soldier and there were two others next to me a medic and an engineer who was maintaining a sentry (lvl 3) then out of no where me the medic and the engineer were all backstabbed by a spy whose disguise never went off and the sentry never flinched I want to know how to report a hacker like that whenever I see them