It has been mentioned in some Ahadith that:

1- People in Paradise speak Arabic[1] and the heavenly Houris also speak Arabic.[2]

2- In a hadith by the prophet he says: "I like Arabs for three reasons: First because I am Arab myself. Second because the Quran is in Arabic, and third because Arabic is the language people speak in Paradise."[3]

3- Imam Sadiq narrates a hadith in which his father said: "Allah did not speak to any prophet but in Arabic."[4]

4- Arabic is the language of Allah[5] and Allah has preferred it over other languages.[6]

5- Also at the beginning of the world, Prophet Adam spoke Arabic but after he disobeyed Allah and ate from the forbidden tree, Allah expelled him from Heaven and its blessings and replaced it with plain land and plantations. He also took Arabic and substituted it with Syriac.[7] After him the first person to speak Arabic was prophet Ismail[8]. In his first confrontation with the king of ancient Egypt, Prophet Yusuf greeted him in Arabic[9] and prophet Soleiman chose it as the language of worship[10]. It is also said that all ministers of Imam Mahdi will speak Arabic despite the fact that none of them are Arab.[11]

6- This language has shown itself as a holy language in most acts of worship and some Islamic mu’amalat (transactions). In Islam the Adhan, Iqama, Prayer and Talbiyya of Hajj (saying Labbaik Allahumma Labbaik…) and Umrah must be performed in Arabic and it is obligatory for all Muslims to learn the Arabic words used in them. Also the Quran is recited in Arabic and other religious texts like the Duas, Ziyarat and Tasbihat are in Arabic.  Even the Islamic beliefs are dictated to one who has just passed away in Arabic (this act is called ‘Talqin’).  In some cases, there are discussions in Islamic fiqh to determine whether or not formulas of certain transactions must be performed in Arabic and if it is obligatory, it can have different levels, for instance, despite the different opinions there are among the ulema regarding important issues like marriage and divorce, all agree that the special formula must be recited and it should be done in Arabic if possible. Although the essence of a contract or transaction lays in the intention behind the formula that is verbalized, but regarding important contracts like marriage or divorce there is said to be a divine connection and this is why in such issues the formula must be recited and it should also be recited in Arabic.[12]

Regardless of the chain of narrators of the mentioned Ahadith and whether or not they have been truly stated by a Masoom, they can be prove that Arabic is a special and chosen language. Moreover we must pay attention that first: The great number of this group of Ahadith leaves no need to scrutinize the chain of narrators and secondly, accepting such an issue isn't something unusual and there is no intellectual or narrative reasoning against it.

In any event, Arabic is the language of the Quran, and Allah has chosen this language to convey his everlasting and global message. Its special features and ability to convey different meanings and concepts can be considered proof for it being chosen by Allah,[13] and it is on this premises that the Quran being in Arabic is mentioned as a means of grace and courtesy from Allah in eleven verses of this holy book.

Related Links:

1- Arabic and Different Forms of Worship, Question 146 (website: 890).

2- The Completeness of Arabic, Question 2480 (website: 2621).