صحيح ابن خزيمه
Sahih Ibn Khuzaymah
جماع أبواب السهو فى الصلاةChapters on Forgetfulness in Prayer
(424) بَابُ ذِكْرِ الْمُصَلِّي يَشُكُّ فِي صَلَاتِهِThe chapter on a praying person doubting his prayer
Hadith 1021
Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) narrates from the Noble Prophet (peace and blessings of Allah be upon him) that he (peace be upon him) said: "When a person (the one praying) forgets and does not know how many units of prayer he has performed, then he should perform two prostrations while he is sitting (in Tashahhud)."
