Sahih Muslim 315
Sahih Muslim · Book 3: The Book of Menstruation · Description of the (fluid) of the man and the woman; the child is created from the water of both of them
This tradition has been narrated by Mu'awyia b. Salim with the same chain of transmitters except for the words:
وَحَدَّثَنِيهِ عَبْدُ اللَّهِ بْنُ عَبْدِ الرَّحْمَنِ الدَّارِمِيُّ، أَخْبَرَنَا يَحْيَى بْنُ حَسَّانَ، حَدَّثَنَا مُعَاوِيَةُ بْنُ سَلاَّمٍ، فِي هَذَا الإِسْنَادِ بِمِثْلِهِ غَيْرَ أَنَّهُ قَالَ كُنْتُ قَاعِدًا عِنْدَ رَسُولِ اللَّهِ صلى الله عليه وسلم وَقَالَ زَائِدَةُ كَبِدِ النُّونِ . وَقَالَ أَذْكَرَ وَآنَثَ . وَلَمْ يَقُلْ أَذْكَرَا وَآنَثَا .
I was sitting beside the Messenger of Allah" and some other minor alterations.