Nasai

641. Commentary: From the previous narrations, it is understood that Bilal (radi Allahu anhu) used to give the first call to prayer (adhan), and Ibn Umm Maktum (radi Allahu anhu) the second. In this narration, it is the opposite: Ibn Umm Maktum (radi Allahu anhu) gave the first adhan and Bilal (radi Allahu anhu) the second. It is possible that, with the permission of the Prophet (sallallahu alayhi wa sallam), they used to alternate turns. It is also possible that in the beginning, Bilal (radi Allahu anhu) used to give the first adhan and Amr ibn Umm Maktum (radi Allahu anhu) the second, and later, the responsibility for the second adhan was assigned to Bilal (radi Allahu anhu) and the first to Amr ibn Umm Maktum (radi Allahu anhu). According to Hafiz Ibn Hajar (rahimahullah), his real name is Amr, although he has also mentioned him as Abdullah with an expression of uncertainty. See: [تقريب التهذيب : 734/1 و 552/2]

Whereas Hafiz Ibn Abd al-Barr and others have claimed that there has been a reversal in this hadith and have said that the correct narration is that of Ibn Umar (radi Allahu anhuma) and others. However, this claim is not correct; rather, the hadith is authentic. For further details, see: [فتح الباري : 103/2] And Allah knows best.