He ordered them to stack up pillows behind his back.

他命令他们在他背后堆放一些枕头。

《柯林斯英汉双解大词典》

When we feel offended by someone, we can tell him to his face instead of complaining behind his back.

如果你觉得被冒犯了，可以当面告诉对方，而不是在他背后抱怨。

youdao

He's always imagining (that) we're talking about him behind his back.

他总是胡乱猜想我们在背后说他的闲话。

《牛津词典》

They only use the expression behind his back.

VOA: special.2009.11.29

And when he traveled to his grandmother's house, he sat in the back of the bus behind a white line.

VOA: special.2010.02.07

He was arrested under the Terrorism Act and handcuffed, with his arms held behind his back.

UNESCO: Of T-shirts and a vegan commemoration��

His soldiers weren't yet calling him the Lost Kauz behind his back, not when this began.

NPR: Excerpt: 'The Good Soldiers'

Chong contorted himself to shift his handcuffed arms from behind his back to the front, Iredale said.

CNN: Student forgotten in cell for 5 days will sue DEA