Sunan Ibn Majah
Sunan Ibn Majah 731
حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ سَعْدِ بْنِ عَمَّارِ بْنِ سَعْدٍ، مُؤَذِّنِ رَسُولِ اللَّهِ ـ صلى الله عليه وسلم ـ حَدَّثَنِي أَبِي عَنْ أَبِيهِ عَنْ جَدِّهِ أَنَّ أَذَانَ بِلاَلٍ كَانَ مَثْنَى مَثْنَى وَإِقَامَتُهُ مُفْرَدَةً .
Unreviewed translation. All hadiths in this collection are available in five languages. Titles and introductions may remain in Arabic.
Hisham ibn 'Ammar narrated to us: 'Abd al-Rahman ibn Sa'd ibn 'Ammar ibn Sa'd, the caller to prayer of the Messenger of Allah (peace and blessings be upon him), narrated to me: My father narrated to me, from his father, from his grandfather, that Bilal's call to prayer was in pairs, while his iqamah was pronounced singly.
Notes and open questions — unreviewed
The source describes the adhan as mathna mathna (in pairs) and the iqamah as mufradah (individually/singly).