Hadith: Bilāl was ordered to pronounce the Adhān in pairs and the Iqāmah in singles

عن أنس بن مالك رضي الله عنه مرفوعاً: «أُمِر بِلاَل أن يَشفَع الأَذَان، ويُوتِر الإِقَامَة».
[صحيح] - [متفق عليه]

Anas ibn Mālik (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) ordered Bilāl to pronounce the Adhān in pairs and the Iqāmah in singles.
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) commanded his Muezzin, Bilāl (may Allah be pleased with him), to pronounce the Adhān in pairs, because it is meant to draw the attention of those who are absent from the mosque, so its phrases are to be said twice. This excludes the Takbīr in the beginning, as it was authentically reported as being said four times, and the word of Tawḥīd (lā ilāha illā Allah) at the end, as it is authentically reported as being said once. The Prophet (may Allah's peace and blessings be upon him) also commanded Bilāl to pronounce the Iqāmah in singles, because it is meant to draw the attention of those in attendance. He was commanded to say each sentence once, except for the Takbīr and "qad qāmat aṣ-ṣalāh" (the prayer has started), as it was authentically reported as being doubled.