The Messenger of Allah ﷺ prayed the afternoon prayer, and a man stood up to pray afterward. Umar saw him and said, "Sit down. The People of the Book were destroyed only because there was no clear separation between their prayers."The Messenger of Allah ﷺ said, "Ibn al-Khattab has spoken well."
حدثنا محمد بن جعفر حدثنا شعبة عن الأزرق بن قيس عن عبد الله بن رباح
أن رسول الله ﷺ صلى العصر فقام رجل يصلي فرآه عمر فقال له اجلس فإنما هلك أهل الكتاب أنه لم يكن لصلاتهم فصل فقال رسول الله ﷺ أحسن ابن الخطاب
Its chain of transmission is sahih. It is found at greater length in Abu Dawud 1/214, no. 1007, in the chapter on prayer, a man performing voluntary prayer, and in Abu Yaala 13/101, no. 7166, and in Abdul-Razzaq 2/432, no. 3973. Haythami said, 2/234, "The men of Ahmad's chain are the men of the Sahih."