(6) The Book of the Times (of Prayer)
كتاب المواقيت
(20) Chapter: What Is Recommended Regarding Delaying 'Isha'
(20) باب مَا يُسْتَحَبُّ مِنْ تَأْخِيرِ الْعِشَاءِ ‏
Sunan an-Nasa'i 534 
It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said:
"Were it not that I would impose too much difficulty on my Ummah, I would have commanded them to delay 'Isha' and to use the Siwak for every prayer."
أَخْبَرَنَا مُحَمَّدُ بْنُ مَنْصُورٍ، قَالَ حَدَّثَنَا سُفْيَانُ، قَالَ حَدَّثَنَا أَبُو الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ لَوْلاَ أَنْ أَشُقَّ عَلَى أُمَّتِي لأَمَرْتُهُمْ بِتَأْخِيرِ الْعِشَاءِ وَبِالسِّوَاكِ عِنْدَ كُلِّ صَلاَةٍ ‏"‏ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 534
In-book reference : Book 6, Hadith 41
English translation : Vol. 1, Book 6, Hadith 535