This excerpt is taken from Sheikh Ghulam Mustafa Zaheer Amanpuri's book Khatm-e-Nubuwwat.
The Issue of the Exception of One Word
It is attributed to Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Prophet Muhammad (peace be upon him) said: أنا خاتم النبيين ، لا نبي بعدي، إلا أن يشاء الله .
"I am the Seal of the Prophets; after me, there is no prophet except whom Allah wills."
Imam Jurqani (may Allah have mercy on him) (543 AH) writes:
هذا استثناء موضوع باطل لا أصل له من حديث أنس ولا حميد، وإنما هو من موضوعات محمد بن سعيد الشامي، المصلوب فى الزندقة، وكان لعنه الله وضاعا كذابا ، فوضع هذا الاستثناء فى هذا الحديث، ودعا الناس إليه، حدثهم يه ليوقع فى قلوبهم الشك، وهذا الاستثناء عند المسلمين كفر وإلحاد وزندقة .
"This exception is fabricated; there is no origin of it in the hadith of Sayyiduna Anas (may Allah be pleased with him) or in the hadith of Hamid Tawil (may Allah have mercy on him). It is among the fabricated narrations of Muhammad bin Saeed al-Shami. Muhammad bin Saeed was a fabricator and liar; he was executed for heresy. May Allah curse him. He fabricated this exception and began to spread it among people. The purpose was to sow seeds of doubt in their hearts. Muslims consider this exception as disbelief, atheism, and heresy."
Reference: (الأباطيل والمناكير والصحاح والمشاهير: 261/1)
Imam Hakim (may Allah have mercy on him) (405 AH) says:
وضع هذا الاستثناء لما كان يدعوا إليه من الإلحاد والزنادقة والدعوة إلى المتنبي .
"He was a follower of a false prophet and an atheist and heretic; therefore, he fabricated this exception."
Reference: (المدخل إلى كتاب الإكليل ص : 52)
Hafiz Dhahabi (may Allah have mercy on him) (748 AH) writes:
مما وضع المصلوب على حميد عن أنس : لا نبي بعدي إلا أن يشاء الله .
"Among the fabricated narrations from Hamid on the authority of Anas is this narration that 'after me there is no prophet except whom Allah wills.'"
Reference: (المغني في الضعفاء : 5553)
Hafiz Ibn al-Jawzi (may Allah have mercy on him) (597 AH) says:
هذا الاستثناء موضوع وضعه محمد بن سعيد .
"This exception is fabricated, and it was fabricated by Muhammad bin Saeed."
Reference: (الموضوعات : 279/1)
Hafiz Abu Zur'ah Ibn al-Iraqi (may Allah have mercy on him) (826 AH) writes:
أما كون نبينا صلى الله عليه وسلم خاتم النبيين، فهو منصوص عليه فى التنزيل، وقال هو : لا نبي بعد بعدي، وهو ثابت فى الصحيحين، وفي تهذيب الآثار لمحمد بن جرير الطبري : لا نبي بعدي إن شاء الله، وهذه الريادة موضوعة، وضعها محمد بن سعيد المصلوب كما قاله الحاكم فى الإكليل، ولو صحت لكان هذا الاستثناء لأجل عيسى، فإن نبوته لم تنقط، وإن كانت شريعته قد نسخت، وتأولها ابن عبد البر فى التمهيد على الرؤيا؛ لأنه لم يبق بعده من أجزاء النبوة غيرها .
"The finality of prophethood of the Messenger of Allah (peace be upon him) is an established matter from Allah. The Prophet (peace be upon him) said: 'There is no prophet after me.' This is a hadith in Sahih Bukhari and Sahih Muslim. In the Tahdhib al-Athar of Muhammad bin Jarir al-Tabari (may Allah have mercy on him), there is a narration with the words: 'There is no prophet after me except whom Allah wills.' This addition is fabricated, and it was fabricated by Muhammad bin Saeed al-Masloub. Imam Hakim (may Allah have mercy on him) has clarified this in Al-Ikleel. Even if this addition were authentic, this exception would relate to the descent of Jesus (peace be upon him), as he is a prophet, although his Shariah has been abrogated. Hafiz Ibn Abd al-Barr (may Allah have mercy on him) interpreted these words in At-Tamhid as a dream, because after the Prophet (peace be upon him), only parts of prophethood remain, such as dreams."
Reference: (الغيث الهامع شرح جمع الجوامع ص : 765)