Bilal would call the adhan, then pause, and when he saw that the Prophet ﷺ had come out, he would call the iqamah for the prayer.
حدثنا عثمان بن أبي شيبة حدثنا شبابة عن إسرائيل عن سماك
كان بلال يؤذن ثم يمهل فإذا رأى النبي ﷺ قد خرج أقام الصلاة
Muslim recorded it in the Book of Mosques, Chapter: When Should People Stand for Prayer?, hadith no. 606, through another route from Simak bin Harb, with different wordings conveying a similar meaning.