Riyad as-Salihin, Book 9, Hadith 1231 (In-Book) ⓘ

Ibn 'Umar (May Allah be pleased with them) reported: The Messenger of Allah had two Mu'adhdhin: Bilal and Ibn Umm Maktum (May Allah be pleased with them). The Messenger of Allah said, "Bilal proclaims Adhan [call to Salat (prayer)] at fag end of night, so eat and drink till Ibn Umm Maktum pronounces Adhan." Ibn 'Umar (May Allah be pleased with them) added: And the gap between their Adhan was not more than the time spent in one's getting down and the other's climbing (the minaret).[Al-Bukhari and Muslim].

وعن ابن عمر رضي الله عنهما قال‏:‏ كان لرسول الله صلى الله عليه وسلم مؤذنان‏:‏ بلال وابن أم مكتوم‏.‏ فقال رسول الله صلى الله عليه وسلم ‏ "‏ إن بلالا يؤذن بليل فكلوا واشربوا حتى يؤذن ابن أم مكتوم‏" ‏ قال ولم يكن بينهما إلا أن ينزل هذا ويرقى هذا، ‏(‏‏(‏متفق عليه‏)‏‏)‏