Sunan Ibn Majah 4204

It was narrated that Abu Sa’eed said:“The Messenger of Allah (ﷺ) came out to us when we were discussing Dajjal (False Christ) and said: ‘Shall I not tell you of that which I fear more for you than Dajjal?’ We said: ‘Yes.’ He said: ‘Hidden polytheism, when a man stands to pray and makes it look good because he sees a man looking at him.’”