*Imam Jafar as-Sadiq (as) has said: "One who offers his prayers along with Adhan and Iqamah, two rows of angels pray behind him, while one who offers his prayers with only the Iqamah and without the Adhan, one row of angels prays behind him." Imam was asked: "And how long is each row?" Imam replied: "At the very minimum, its length is the distance between East and West, while at the maximum, its length is the distance between the earth and the heavens." (Wasail ash-Shia, vol. 4, p. 620)

*The general secret of the Adhan is the announcement to the overt and covert powers and the divine armies to attend the Meeting. Its general discipline is to be aware of the greatness of the position, its significance and the majesty of the Presence and the Present, and it is the servility, helplessness, poverty, incapability and shortcoming of the "possible" (existent) in carrying out orders and deserving to attend the Meeting [mahdar], unless the kindness and mercy of Allah, the Most High, extend the helping hand to make up for the shortcomings. Adabus Salaat - Imam Khomeini.

*From https://www.duas.org/calltoprayer.htm