Sunan an-Nasa'i
Book 1: The Book of Purification
كتاب الطهارة
325 hadith in this book
Chapter 126 Mentioning When Ghusl (A Purifying Bath) Is Obligatory And When It Is Not - The Ghusl Of The Disbeliever when He Accepts Islam
باب ذِكْرِ مَا يُوجِبُ الْغُسْلَ وَمَا لاَ يُوجِبُهُ - غُسْلُ الْكَافِرِ إِذَا أَسْلَمَ
It was narrated from Qais bin 'Asim that he accepted Islam, and the Prophet (ﷺ) commanded him to perform Ghusl with water and lotus leaves.
أَخْبَرَنَا عَمْرُو بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا يَحْيَى، قَالَ حَدَّثَنَا سُفْيَانُ، عَنِ الأَغَرِّ، - وَهُوَ ابْنُ الصَّبَّاحِ - عَنْ خَلِيفَةَ بْنِ حُصَيْنٍ، عَنْ قَيْسِ بْنِ عَاصِمٍ، أَنَّهُ أَسْلَمَ فَأَمَرَهُ النَّبِيُّ صلى الله عليه وسلم أَنْ يَغْتَسِلَ بِمَاءٍ وَسِدْرٍ .
Hadith text, translations and grading are courtesy of Sunnah.com.