The Book of Funerals
21/34. Chapter:
Washing the Deceased more than seven times
Sunan an-Nasai
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا حَمَّادٌ، عَنْ أَيُّوبَ، عَنْ حَفْصَةَ، عَنْ أُمِّ عَطِيَّةَ، نَحْوَهُ غَيْرَ أَنَّهُ قَالَ " ثَلاَثًا أَوْ خَمْسًا أَوْ سَبْعًا أَوْ أَكْثَرَ مِنْ ذَلِكِ إِنْ رَأَيْتُنَّ ذَلِكِ " .
Something similar was narrated from Umm 'Atiyyah except, that he (the narrator) said: "Three times or five, or seven, or more than that, if you think that (is necessary)."
Reference : Sunan an-Nasa'i 1888In-book reference : Book 21, Hadith 71English translation : Vol. 3, Book 21, Hadith 1889