All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.

 

It isn`t forbidden for a Muslim to call his eldest son "Qasim" since there is no evidence in Sharia that this forbidden. However, according to the reliable opinion of our doctrine (Shafi), it is forbidden to call one (Other than Allah`s Apostle) with the Kunyah of Abul-Qasim because it was narrated from Jabir that the Messenger of Allah (Peace and blessings be upon him) said: "Call yourselves by my name but do not call yourselves by my Kunyah." {Agreed upon}.

Therefore, whoever wants to call his eldest son "Qasim" or "Al-Qasim" should choose for himself a Kunyah other than "Abul-Qasim." In addition, it is well known that one`s Kunyah doesn`t have to take the name of his eldest son since the Kunyah of Omar Bin Al-Khattab (May Allah be pleased with him) was "Abu Hafs" although none of his children was named "Hafs."

However, this is a controversial issue amongst the learned men of the faith.

An-Nawawi (May Allah have mercy on him) said: "Scholars disagreed on calling one`s-self "Abul-Qasim." According to the Shafie school of thought, it isn`t lawful for anyone to be called with the Kunyah of Abul-Qasim, whether his name was Mohammad or not. This is based on the apparent meaning of the above Hadith. Amongst the trustworthy fellow scholars who reported this opinion from Al-Shafie are: Abu baker al-Baihaqhi, Abu Mohammad Al-Baghwi and Abukasim Ibn Asaker.

According to the Maliki school of thought, it is permissible for one whose name is or isn`t Mohammad to be called with the Kunyah of Abul-Qasim. However, the forbiddance in calling one with the Kunya of abul-Qasim is special for the Prophet Mohammad (Peace and blessings be upon him) during his life time.

As for the third opinion on this issue, it isn`t permissible for the one whose name is Mohammad to be called with the Kunya of Abul-Qasim while this is permissible for the one whose name isn`t Mohammad. 

In the book of Nikah (Marriage), Al-Rafe`i said that he opts for the third opinion because people in different ages are still calling one another with the Kunya of Abul-Qasim, and none of the scholars have objected to that. This third opinion clearly contradicts the apparent meaning of the above Hadith. As for people`s agreement on calling one another with this Kunya, it is based on the fact that this is practiced by the eminent scholars themselves and the people of authority. This, in fact, gives preponderance to the view of Imam Malik and that the forbiddance in calling one with the Kunya of Abul-Qasim is special for the Prophet Mohammad (Peace and blessings be upon him) during his life time. This is also because the Jews used to call themselves with this Kunya to harm the Prophet (Peace and blessings be upon him), but now the Prophet (Peace and blessings be upon him) is dead, so there is no meaning for this forbiddance." {Al-Majmou`, vol.8/pp.439-440}. For more details, kindly check {Moghni Al-Mohtaaj, vol.1/pp.35} by Al-Shirbini. And Allah The Almighty Knows Best.