Mishkat al-Masabih 4 Prayer كتاب الصلاة 517

عَنْ أَنَسٍ قَالَ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُول: «اسْتَووا اسْتَوُوا اسْتَوُوا فَوَالَّذِي نَفْسِي بِيَدِهِ إِنِّي لَأَرَاكُمْ من خَلْفي كَمَا أَرَاكُم من بَين يَدي» . رَوَاهُ أَبُو دَاوُد

Anas told how the Prophet used to say, ‘‘Keep in alignment, keep in alignment, keep in alignment, for by Him in whose hand my soul is, I can see you behind me just as I see you in front of me.” Abu Dawud transmitted it.