صحيح ابن خزيمه
Sahih Ibn Khuzaymah
جماع أبواب السهو فى الصلاةChapters on Forgetfulness in Prayer
(422) بَابُ التَّسْلِيمِ مِنَ الرَّكْعَتَيْنِ سَاهِيًا فِي الظُّهْرِ أَوِ الْعَصْرِ أَوِ الْعِشَاءِThe chapter on mistakenly ending the prayer after two units in Dhuhr, Asr, or Isha
Hadith Q1034
وَإِبَاحَةِ الْبِنَاءِ عَلَى مَا قَدْ صَلَّى الْمُصَلِّي قَبْلَ تَسْلِيمِهِ فِي الرَّكْعَتَيْنِ سَاهِيًا، وَالدَّلِيلِ عَلَى أَنَّ السَّلَامَ سَاهِيًا قَبْلَ الْفَرَاغِ مِنَ الصَّلَاةِ لَا تَفْسَدُ الصَّلَاةُ
It is permissible for the worshipper to build upon what he has already prayed if he forgetfully says the salam after two rak‘ahs before completing the prayer. And the explanation of the evidence that forgetfully saying the salam before finishing the prayer does not invalidate the prayer.
