According to some of the Islamic Scholars, Arabic Bible was not available during lifetime of Prophet (sws). It became available after the demise of Prophet Muhammad (sws). If this is true, then which Bible were the Arab Christians reading before and during lifetime of Prophet (sws)?
I am not an expert in ancient linguistic but based on some research it seems like the Christians of Najran had their Bible in Arabic language with Musnad alphabets that was the alphabet used in most areas of South Arabia before it was replaced with the modern Arabic alphabet.
In his article on the origins of the name "Allah", titled "Who was 'Allah' before Islam", Rick Brown writes:
"As for the Arab Christians, although some of those in north-western Arabia were Greek Orthodox, the historical records indicate that many or most of the Arab Christians belonged to the Nestorian and Monophysite churches and that their liturgy and Scriptures were in Syriac, a variety of Aramaic. Most of the common-era pre-Islamic inscriptions found in Arabia are written in varieties of Aramaic, although there are also inscriptions in Greek, Arabic, and South Arabian."
When referring to the Christians of Najran he refers to the book "The Martyrs of Najran" by Irfan Shahid (1971) and writes:
"Irfan Shahid (1971: 249–250) presents evidence that before 520 AD the Christians of Najran had the Gospel in their language, meaning their dialect of Arabic, written in Musnad script."
For more information on Musnad alphabets please refer to this source: