Sunan an-Nasa'i, Book 10, Hadith 50 (In-Book) ⓘ
It was narrated that 'Abdullah bin 'Umar said: "The Messenger of Allah used to enjoin upon us to make the prayer short, but he would lead us in prayer and recite As-Saffat."
أَخْبَرَنَا إِسْمَاعِيلُ بْنُ مَسْعُودٍ، قَالَ حَدَّثَنَا خَالِدُ بْنُ الْحَارِثِ، عَنِ ابْنِ أَبِي ذِئْبٍ، قَالَ أَخْبَرَنِي الْحَارِثُ بْنُ عَبْدِ الرَّحْمَنِ، عَنْ سَالِمِ بْنِ عَبْدِ اللَّهِ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَأْمُرُ بِالتَّخْفِيفِ وَيَؤُمُّنَا بِالصَّافَّاتِ .
also referenced under: Hadith 826 (Reference) & Volume 1, Book 10, Hadith 827 (DarusSalam)