وَمَنْ جَعَلَ مِنْهُمْ ظَهْرَهُ إِلَى وَجْهِ الْإِمَامِ: لَمْ تَجُزْ صَلَاتُهُ.
Translation: If a follower places his back toward the imam’s face, his prayer is not valid.
This describes the follower being ahead of the imam in the relevant direction. The problem is not merely whether he looks toward or away from the imam; it is the spatial precedence of the follower.
This must not be confused with the valid back-to-back arrangement, where the positions and directions are different.