The iqamah is not to be pronounced until the imam has stepped into the mosque.
This is based on the hadith of Jabir ibn Samurah: "The Prophet’s ﷺ mu’adhdhin would call the adhan and then wait, refraining from the iqamah until he saw that the Messenger of Allah ﷺ had come out; when he saw him, he would pronounce the iqamah."
It was recorded by Abu Dawud, al-Tirmidhi, and Ahmad through the chain of Isra’il, who said: “Simmak informed me that he heard Jabir ibn Samurah relate it.” Al-Tirmidhi commented: “A good and sound (hasan sahih) hadith.”
I say: The chain meets Muslim’s conditions, and he included it in his Sahih through another route from Simmak; its wording has already been cited under the adhan, issue (13).

Likewise, the congregation should not stand up until they see that the imam has come out, even if the iqamah has already been given, because the Prophet ﷺ said: “When the iqamah is called, do not stand until you see me [have come out]; and remain calm.”
This hadith is narrated by Abu Qatadah al-Ansari (may Allah be pleased with him).
It is reported by al-Bukhari, Muslim, Abu Dawud, al-Nasa’i, al-Tirmidhi, al-Darimi, al-Bayhaqi, and Ahmad through several routes from Yahya ibn Abi Kathir, from ‘Abdullah ibn Abi Qatadah, from his father.
The first additional phrase is the version of al-Tirmidhi; it is likewise the version found in Muslim, Abu Dawud, and al-Nasa’i.