Sunan an-Nasa'i, Volume 1, Book 7, Hadith 628 (DarusSalam) ⓘ
It was narrated that Anas said: "The Messenger of Allah commanded Bilal to say the phrases of the Adhan twice and the phrases of the Iqamah once."
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا عَبْدُ الْوَهَّابِ، عَنْ أَيُّوبَ، عَنْ أَبِي قِلاَبَةَ، عَنْ أَنَسٍ، قَالَ إِنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم أَمَرَ بِلاَلاً أَنْ يَشْفَعَ الأَذَانَ وَأَنْ يُوتِرَ الإِقَامَةَ .
also referenced under: Book 7, Hadith 634 (In-Book)