Look for something by which you can announce the time for prayer as a sign for it. So Bilal was ordered to pronounce the adhan in pairs and the iqamah singly.
حدثنا عبد الله بن الجراح حدثنا المعتمر بن سليمان عن خالد الحذاء عن أبي قلابة
التمسوا شيئا يؤذنون به علما للصلاة فأمر بلال أن يشفع الأذان ويوتر الإقامة
Bukhari transmitted it in the Book of Adhan, chapter on the beginning of the adhan, hadith no. 603, and Muslim in the Book of Prayer, chapter on the command to say the adhan in pairs and the iqamah singly except for the phrase of the iqamah, which is doubled, hadith no. 378, both from Khalid Haddan with this same chain.