The Book of Purification
1/7. Chapter:
Permitting The Usage Of Siwak In The Afternoon For One Who Is Fasting
Sunan an-Nasai
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، عَنْ مَالِكٍ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " لَوْلاَ أَنْ أَشُقَّ عَلَى أُمَّتِي لأَمَرْتُهُمْ بِالسِّوَاكِ عِنْدَ كُلِّ صَلاَةٍ " .
It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said: "Were it not that it would be too difficult for my Ummah, I would have commanded them to use the Siwak at (the time of) every Salah."
Reference : Sunan an-Nasa'i 7In-book reference : Book 1, Hadith 7English translation : Vol. 1, Book 1, Hadith 7