Ibn 'Umar (May Allah be pleased with them) reported:
The Messenger of Allah ﷺ had two Mu'adhdhin: Bilal and Ibn Umm Maktum (May Allah be pleased with them). The Messenger of Allah ﷺ 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 ﷺ مؤذنان: بلال وابن أم مكتوم. فقال رسول الله ﷺ " إن بلالا يؤذن بليل فكلوا واشربوا حتى يؤذن ابن أم مكتوم" قال ولم يكن بينهما إلا أن ينزل هذا ويرقى هذا، ((متفق عليه))