كتاب الصلاة
4  
The Book of Prayers
(51)
Chapter: Lying In Front Of One Who Is Praying
(51)
باب الاِعْتِرَاضِ بَيْنَ يَدَىِ الْمُصَلِّي ‏
Sahih Muslim 514

'A'isha reported:

The Apostle of Allah (ﷺ) said prayer at night and I was by his side in a state of meanses and I had a sheet pulled over me a portion of which was on his side.
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَزُهَيْرُ بْنُ حَرْبٍ، قَالَ زُهَيْرٌ حَدَّثَنَا وَكِيعٌ، حَدَّثَنَا طَلْحَةُ بْنُ يَحْيَى، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ، قَالَ سَمِعْتُهُ عَنْ عَائِشَةَ، قَالَتْ كَانَ النَّبِيُّ صلى الله عليه وسلم يُصَلِّي مِنَ اللَّيْلِ وَأَنَا إِلَى جَنْبِهِ وَأَنَا حَائِضٌ وَعَلَىَّ مِرْطٌ وَعَلَيْهِ بَعْضُهُ إِلَى جَنْبِهِ ‏.‏
Reference : Sahih Muslim 514
In-book reference : Book 4, Hadith 309
USC-MSA web (English) reference : Book 4, Hadith 1042
  (deprecated numbering scheme)