Sunnah.com reference : Book 2, Hadith 37Arabic/English book reference : Book 2, Hadith 181
Narrated Anas (RA):
Bilal was commanded to announce the Adhan (each phrase) twice and the Iqamah (each phrase) once except "The prayer stood ready to begin" (to be pronounced twice).

[Agreed upon; but Muslim did not mentioned the exception].
وَعَنْ أَنَسِ]بْنِ مَالِكٍ] ‏- رضى الله عنه ‏- قَالَ: { أُمِرَ بِلَالٌ أَنْ يَشْفَعَ اَلْآذَانَ, وَيُوتِرَ اَلْإِقَامَةَ, إِلَّا اَلْإِقَامَةَ, يَعْنِي قَوْلَهُ: قَدْ قَامَتِ اَلصَّلَاةُ }  مُتَّفَقٌ عَلَيْهِ, وَلَمْ يَذْكُرْ مُسْلِمٌ اَلِاسْتِثْنَاءَ  1‏ .‏‏1 ‏- صحيح.‏ رواه البخاري (605)‏، ومسلم (378)‏.‏