It is Authentic, it has been narrated in a number of books. It has been narrated in Sunnan Annasa'i, and is Hasan. Other books in which it has been narrated are: Abi Dawood in which it is Da'eef, and in Jami'i Tirmidhi, in which the hadith is saheeh:
إِنَّ اللَّهَ أَمَرَكُمْ بِالصَّلاَةِ فَإِذَا صَلَّيْتُمْ فَلاَ
تَلْتَفِتُوا فَإِنَّ اللَّهَ يَنْصِبُ وَجْهَهُ لِوَجْهِ عَبْدِهِ فِي
صَلاَتِهِ مَا لَمْ يَلْتَفِتْ
Allah commands you to perform Salat, and when you perform Salat then
do not turn away, for Allah is facing the face of His worshipers as
long as he does not turn away.
And there are similar Ahadeeth in regards to spitting in front of your during salat and others, which I will not mention in my answer to this question.