صحيح ابن خزيمه
Sahih Ibn Khuzaymah
جماع أبواب الأفعال المباحة فى الصلاةChapters on Permissible Acts in Prayer
(327) بَابُ الرُّخْصَةِ فِي بَصْقِ الْمُصَلِّي خَلْفَهُ، و327 Chapter: Permission for the Worshipper to Spit Behind Him.
Hadith Q876
َفِيهِ مَا دَلَّ عَلَى إِبَاحَةِ لَيِّ الْمُصَلِّي عُنُقَهُ وَرَاءَ ظَهْرِهِ إِذَا أَرَادَ أَنْ يَبْصُقَ فِي صَلَاتِهِ، إِذِ الْبَزْقُ خَلْفَهُ غَيْرُ مُمْكِنٍ إِلَّا بِلَيِّ الْعُنُقِ
And in this is evidence that it is permissible for the one praying to turn his neck backwards when he intends to spit, because spitting backwards is not possible without turning the neck.
