A Sufi Shaykh claim of Tahqiq

Q: Our Shaykh often swears and says that according to his research, the Hadith about the Prophet (peace be upon him), "Allah has forgiven all your sins, the past and future ones" is definitely not a Hadith. He adds that this Hadith had been altered and changed, and that it is an interpolation of the Jews, despite its existence in the Sahih (authentic) books of Hadith, such as Al-Bukhari and others.

A: In the Noble Qur'an, Allah mentions that He has forgiven for His Prophet (peace be upon him) his past and future sins. Allah (Exalted be He) says: Verily, We have given you (O Muhammad صلى الله عليه وسلم) a manifest victory. That Allâh may forgive you your sins of the past and the future, and complete His Favour on you, and guide you on the Straight Path, (Part No. 2; Page No. 64) It was proven: "The Prophet (peace be upon him) used to offer Prayer at night (for such a long time) that his feet used to crack. `Aishah said, 'O Messenger of Allah! Why do you do this since Allah has forgiven for you your past and future sins?' He said, 'Should I not love to be a thankful Slave (of Allah)?'" This is an honor to the Messenger of Allah (peace be upon him), and it clarifies his status, for he is the best human being and the master of those who have come before and after him. Therefore, if anyone denies this after the proof has been established for them, they will be Kafirs (disbelievers) for denying a definite issue and disobeying the Noble Qur'an and the purified Sunnah (whatever is reported from the Prophet).