كتاب الحيض
3  
The Book of Menstruation
(13)
Chapter: It is recommended for the woman who is performing ghusl following menses to apply a piece of cloth scented with musk to the site of the bleeding
(13)
باب اسْتِحْبَابِ اسْتِعْمَالِ الْمُغْتَسِلَةِ مِنَ الْحَيْضِ فِرْصَةً مِنْ مِسْكٍ فِي مَوْضِعِ الدَّمِ ‏
Sahih Muslim 332 d

This hadith is narrated by 'Ubaidullah b. Mu'adh with the same chain of transmitters (but for the words) that he (the Holy Prophet) said:

Cleanse yourself with it, and he covered (his face on account of shyness).
وَحَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا شُعْبَةُ، فِي هَذَا الإِسْنَادِ نَحْوَهُ وَقَالَ قَالَ ‏ "‏ سُبْحَانَ اللَّهِ تَطَهَّرِي بِهَا ‏"‏ ‏.‏ وَاسْتَتَرَ ‏.‏
Reference : Sahih Muslim 332d
In-book reference : Book 3, Hadith 73
USC-MSA web (English) reference : Book 3, Hadith 650
  (deprecated numbering scheme)