Mishkat al-Masabih 1135
Ibn ‘Umar said:
God's Messenger used to command us to be brief, and he would recite as-Saffat (Al-Qur’an; 37) when he was our imam . 

Nasa’i transmitted it.
وَعَنِ ابْنِ عُمَرَ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَأْمُرُنَا بِالتَّخْفِيفِ وَيَؤُمُّنَا ب (الصافات)
رَوَاهُ النَّسَائِيّ