How sound is the Hadith: "Hold fast to my Sunnah and the Sunnah of the Rightly Guided Caliphs"? Is it permissible for anyone after the Messenger of Allah (peace be upon him) to establish a Sunnah for Muslims, knowing that the Messenger does not speak out of his own desire? And did the Messenger of Allah (peace be upon him) specify the Rightly Guided Caliphs in a particular Hadith, and who are they? If so, why is there disagreement regarding the non-specification of the Caliphate?

The hadith is authentic. It was authenticated by Al-Tirmidhi and Al-Hakim, and Al-Dhahabi concurred with them. It is not permissible to innovate anything that contradicts the guidance of the Prophet, peace and blessings be upon him. The Caliphs adhered firmly to his guidance, and they exercised ijtihad (independent reasoning) when there was no explicit prophetic text. Al-Mubarakpuri mentioned that the Caliphs acted only according to the Sunnah of the Prophet, peace and blessings be upon him. Al-Qari identified the Caliphs as the four: Abu Bakr, Umar, Uthman, and Ali, citing as evidence the hadith: "The Caliphate after me will be thirty years." Some scholars included Al-Hasan ibn Ali among them. However, their specific designation has not been established in an authentic hadith; rather, what was reported is: "Follow those who come after me: Abu Bakr and Umar," and this is an authentic hadith.