(4) The Book of Ghusl and Tayammum
كتاب الغسل والتيمم
(7) Chapter: Concealing Oneself When Performing Ghusl
(7) باب الاِسْتِتَارِ عِنْدَ الاِغْتِسَالِ
Sunan an-Nasa'i 408 
It was narrated that Maimunah said:
"I put some water out for the Messenger of Allah (ﷺ), then I concealed him" - and she mentioned how he performed Ghusl, then she said: "Then I brought him a cloth (a towel) but he did not want it."
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا عَبِيدَةُ، عَنِ الأَعْمَشِ، عَنْ سَالِمٍ، عَنْ كُرَيْبٍ، عَنِ ابْنِ عَبَّاسٍ، عَنْ مَيْمُونَةَ، قَالَتْ وَضَعْتُ لِرَسُولِ اللَّهِ صلى الله عليه وسلم مَاءً - قَالَتْ - فَسَتَرْتُهُ فَذَكَرَتِ الْغُسْلَ قَالَتْ ثُمَّ أَتَيْتُهُ بِخِرْقَةٍ فَلَمْ يُرِدْهَا ‏‏.‏‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 408
In-book reference : Book 4, Hadith 13
English translation : Vol. 1, Book 4, Hadith 408