This was a Roman custom with defeated enemies.
Colossians 2:14-16 New International Version 14 having canceled the charge of our legal indebtedness, which stood against us and condemned us; he has taken it away, nailing it to the cross.
15 And having disarmed the powers and authorities,
he made a public spectacle of them, triumphing over them by the cross.
I think this is the nature of Trump's "praise allah" comment.