Why was Havertz's goal disallowed?

Immediately, the referee disallowed the goal.

But why?

Well, it's a rule that we hardly ever see in football.

According to the rules, "The kicker must not play the ball again until it has touched another player."

That means that, even if it hits the post, the same player can't touch it until it's been touched by another player.

The same applies to a player who slips and accidentally kicks the ball twice, as we saw with Aleksandar Mitrovic's penalty for Fulham against Newcastle in January.

Source : givemesport