Musnad Ahmad 32:1559

Hassan ibn Musa narrated to us, Amara—meaning Ibn Zadhan—narrated to us, Thabit narrated to us, from Anas ibn Malik: "The mu'adhdhin—or Bilal—would perform the iqamah, and the Prophet (peace and blessings of Allah be upon him) would enter. A man would meet him regarding a need, and he would stand with him until most of them would nod their heads [in sleep]."