From Sahih Muslim (Hadith 833):
Anas reported:(The people) stood up for prayer and the Messenger of Allah (ﷺ) was whispering to a man, and in the narration of 'Abd al-Warith (the words are): The Apostle of Allah (ﷺ) was having a private conversation with a man, and did not start the prayer till the people dozed off
Reference: Muslim 833