صحيح ابن خزيمه
Sahih Ibn Khuzaymah
جماع أبواب قيام المأمومين خلف الإمام وما فيه من السننChapters on Followers Standing Behind the Imam
(122) بَابُ إِدْرَاكِ الْمَأْمُومِ الْإِمَامَ سَاجِدًا، وَالْأَمْرِ بِالِاقْتِدَاءِ بِهِ فِي السُّجُودِ،122| Chapter on the follower catching up with the imam in prostration and the command to follow him in prostration
Hadith Q1622
وَأَنْ لَا يَعْتَدَّ بِهِ إِذِ الْمُدْرِكِ لِلسَّجْدَةِ إِنَّمَا يَكُونُ بِإِدْرَاكِ الرُّكُوعِ قَبْلَهَا.
And he should not count this prostration, because the one who catches the prostration is only the one who has also caught the bowing before it (otherwise, a single prostration will not complete the rak'ah).
