Why is the speech of the Prophet, peace be upon him, considered a proof?

The Hadith is what has been transmitted from the Messenger of Allah, peace and blessings be upon him, whether it is a saying, an action, an approval, or a description. It either confirms what is in the Quran, or elaborates on what it generalized, or clarifies a ruling about which the Quran was silent. Allah commanded His Prophet to clarify what was revealed to him, and his revelation is a revelation from Allah. Allah sent him as a mercy to the worlds, and he was keen on guiding the Ummah and conveying messages from his Lord. Obedience to him is obligatory; it is obedience to Allah and the path to salvation and success. Whoever obeys him will succeed, and whoever disobeys him will go astray. Faith is not complete without loving him and following him, and love necessitates obedience and belief. Allah has perfected the religion, and the Messenger has conveyed his message, and the Companions preserved his Hadiths. Every innovation in religion is rejected, as the Prophet, peace and blessings be upon him, said: "Whoever introduces into this affair of ours anything that does not belong to it will have it rejected."