كتاب الحيض
3  
The Book of Menstruation
(17)
Chapter: The prohibition of looking at `awrah
(17)
باب تَحْرِيمِ النَّظَرِ إِلَى الْعَوْرَاتِ ‏
Sahih Muslim 338 b

This hadith has been narrated by Ibn Abu Fudaik and Dabbik b. 'Uthman with the same chain of transmitters and they observed:

Private parts of man are the nakedness (which is concealed).
وَحَدَّثَنِيهِ هَارُونُ بْنُ عَبْدِ اللَّهِ، وَمُحَمَّدُ بْنُ رَافِعٍ، قَالاَ حَدَّثَنَا ابْنُ أَبِي فُدَيْكٍ، أَخْبَرَنَا الضَّحَّاكُ بْنُ عُثْمَانَ، بِهَذَا الإِسْنَادِ وَقَالاَ - مَكَانَ عَوْرَةِ - عُرْيَةِ الرَّجُلِ وَعُرْيَةِ الْمَرْأَةِ ‏.‏
Reference : Sahih Muslim 338b
In-book reference : Book 3, Hadith 91
USC-MSA web (English) reference : Book 3, Hadith 668
  (deprecated numbering scheme)