The Prophet ﷺ said to the people, "Perfect your prayer, for I see you from behind me just as I see you from in front of me."
حدثنا قتيبة حدثنا ليث بن سعد عن ابن عجلان عن أبيه
أن النبي ﷺ قال للناس أحسنوا صلاتكم فإني أراكم من خلفي كما أراكم أمامي
No references are available.
Its isnad is sahih, and it was narrated by the six major collections with similar wording. Bukhari 2/325 no. 741 Fath, in the Book of Adhan, chapter on humility in prayer, and Muslim 1/319 no. 424, in the Book of Prayer, chapter on the command to perform prayer well.