The first whose tongue Allah opened to speak clear Arabic was Ismail.
أول من فتق الله لسانه بالعربية المبينة إسماعيل.
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Zubayr bin Bakkar narrated it in Nasab with a hasan chain. Ibn Hajar stated this in Fath al-Bari (6/403), though I have not been able to locate its chain myself, and I have only graded it hasan based on Ibn Hajar's citation and his grading of it as such.Umawi said, Ali bin Mughira told me, Abu Ubayda told us, Musmi bin Malik told us, from Muhammad bin Ali bin Husayn, from his forefathers, from the Prophet ﷺ, and he mentioned something similar. Bidayah wa Nihayah (1/444). This chain is mursal, meaning it has a break.His statement, "He was the first to loosen his tongue with Arabic," means he was the first to speak Arabic in the Hijaz and its surrounding areas, because Arabic was the language of the people of Yemen, and Ismail learned it from them when they settled in Makkah around Zamzam, as mentioned in the words of Ibn Abbas in Sahih Bukhari (3364). As for the language of the people of the Hijaz and those around them, it is not known precisely, and perhaps they spoke one of the Semitic languages that has since become extinct. And Allah knows best.It was narrated from Ibn Abbas that he said, "The first to speak Arabic, and who devised writing according to its pronunciation and speech, then made it a single connected script, like writing 'In the name of Allah, the Most Merciful, the Most Compassionate' joined together, until his son Ismail bin Ibrahim, peace be upon them both, separated it."Hakim narrated it (2/553), and Bayhaqi narrated it through his route in Shuab al-Iman (1617), from Husayn bin Hasan bin Ayub, Abu Yahya bin Abu Maysara told us, Ibrahim bin Munzir Hizami told us, Abdul-Aziz bin Imran told me, Ismail bin Ibrahim bin Abu Habiba told me, from Dawud bin Husayn, from Ikrima, from Ibn Abbas, and he related it.In its chain is Dawud bin Husayn, who is trustworthy except in his narration from Ikrima, where his hadith is considered rejected, as Ibn Madini, Abu Dawud, and others said, and this is one such instance. Ibn Adi said, "His hadith is acceptable, when a trustworthy narrator transmits from him, his narration is acceptable, but if a daif narrator transmits from him, then the flaw lies with that narrator, such as Ibn Abu Habiba and Ibrahim bin Abu Yahya." See Tahzib Kamal (2/412).I say, this hadith comes through the narration of Ibn Abu Habiba, from him, from Ikrima. Ibn Abu Habiba is Ibrahim bin Ismail bin Abu Habiba al-Ashhali. A transposition occurred in his name, so it became Ismail bin Ibrahim bin Abu Habiba, but the correct form is what appears in Jarh wa Tadil (2/83) and Tahzib Kamal (1/100), namely Ibrahim bin Ismail bin Abu Habiba.This Ibrahim bin Ismail was deemed trustworthy by Ahmad bin Hanbal, though most scholars criticized him, and the conclusion is that he is daif, as Ibn Hajar said in Taqrib.Hakim graded this hadith sahih, saying, "This is a hadith with a sahih chain, and the two of them did not record it." Dhahabi responded to him by saying, "Abdul-Aziz is weak."I say, it is as he said. Abdul-Aziz is Ibn Imran al-Zuhri, whose narrations are abandoned. His books were burned, so he narrated from memory, which greatly increased his errors, though he was knowledgeable about genealogies, as Ibn Hajar said.