Jabir bin Samurah said: Bilal would call the Adhan, then he used come to wait. When he would see that the prophet ﷺ had come out (of his house), he would pronounce the iqama.
Hadith Referenceسنن ابي داود / كتاب الصلاة / 537
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:صحيح مسلم (606)
It is thus established that it is not necessary for the imam to be standing on his prayer place (musalla) before the iqamah is pronounced; rather, it is permissible to pronounce the takbir (iqamah) upon seeing the imam approaching as well.
Explanation & Benefits
Shaykh Umar Farooq Saeedi
537. Commentary: It is not necessary for the imam to be already standing on his prayer mat before the iqamah is called; rather, it is permissible to pronounce the takbir (iqamah) upon seeing him approaching as well.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 537
´Jabir bin Samurah narrated:` The Mu'adh-dhin of Allah's Messenger would wait and he would not call the Iqamah until he saw the Allah's Messenger had come out, he would call the Iqamah when he saw him."
Shaykh Dr. Abdur Rahman Freywai
1: Because the muezzin has been appointed as the guardian of the time of the call to prayer (adhan), therefore, no one has the right to compel him to delay or advance the adhan.
2: Therefore, without his indication or permission, the iqamah (second call to prayer) should not be pronounced.
Source: Sunan al-Tirmidhi – Majlis ‘Ilmi Dar al-Da‘wah, New Delhi Edition, Page: 202