Musnad Ahmad 30:3802

Hashim narrated to us, Mas'udi and Sharik narrated to us, from Ash'ath ibn Abi al-Sha'tha', from his father, from Abu Hurayrah, who said: A man went out of the mosque after the caller had given the adhan. He said: As for this one, he has disobeyed Abu al-Qasim, may Allah bless him and grant him peace. He said: And in the hadith of Sharik: Then he said: The Messenger of Allah, may Allah bless him and grant him peace, commanded us: "If you are in the mosque and the call to prayer is made, let not one of you go out until he prays."