Ja'd Abu 'Uthman says that Hazrat Anas entered a mosque, the people had already performed the prayer, he ordered a man who then gave the adhan and the iqamah.

Hazrat Tawus, Hazrat Ata, and Hazrat Mujahid say that when you enter a mosque and whether the prayer has been performed there or not, you should say the Iqamah before offering the prayer.

Hazrat Zuhri says that he should give the adhan as well as say the iqamah.

Hazrat Saeed bin Musayyib says that if some people go to the mosque and the prayer has already been performed there, then they should also give the adhan and say the iqamah. Hazrat Qatadah says that the declaration of the oneness of Allah and the prophethood of the Holy Prophet (peace be upon him) will surely bring goodness.