Shuaib bin Abi Hamzah said: Ibn al-Munkadir, Ibn Abi Farwah and a number of jurists of Madina said to me: When you recite the supplication “I am first of the Muslims, ” say instead; “I am one of the Muslims”.
Hadith Referenceسنن ابي داود / أبواب تفريع استفتاح الصلاة / 762
Hadith Gradingالألبانی:صحيح مقطوع | زبیر علی زئی:إسناده صحيح
762. Commentary: Its explanation has been provided in the benefits of Sunan Abi Dawood, Hadith number: [760], that there is no harm in saying «وَأَنَاأَوَّلُالْمُسْلِمِينَ». Its meaning is: “O Allah! In obeying Your commands, I am at the forefront.” Just as the noble verse states: «قل ان كان للرحمن ولد فانا اول العابدين»[لزخرف : 81] “Say: If (hypothetically) the Most Merciful had a son, then I would be the first to worship him.” Prophet Musa (alayhis salam) said: «وانا اول المؤمنين»[الاعراف : 143] “I am the foremost among the believers.”
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 762