السلام عليكم Why can we make a difference between Hadith Qudsi and normal Ahadith? Or just day to day conversations of the Prophet? What's the evidence for this since in Surah Najm it says he doesn't speak of his whims and it's only revelation sent down to him?

Imam Ibn Bāz said: The qudsī hadith is attributed to Allah — Allah Jalla wa ʿAlā (the Majestic and Exalted) speaks it — however, the Prophet ﷺ transmits it from his Rabb (Lord), and so it is called qudsī. As for what the Prophet ﷺ does not relate directly from his Rabb (Lord), it is called a hadith of the Prophet ﷺ — such as "Actions are but by intentions" and "Islam is built upon five" — these are the words of the Prophet ﷺ. Q: Meaning its sense is from Allah, while its wording is from the Prophet ﷺ? A: It is revelation from Allah, but its words are the words of the Prophet ﷺ. Allah revealed its meaning to him, and the Prophet ﷺ expressed it in speech. As for the qudsī hadith, it is like: "Allah Jalla wa ʿAlā (the Majestic and Exalted) says: O son of Adam, such and such… I have forbidden oppression upon Myself and made it forbidden among you, so do not oppress one another. O son of Adam, all of you are astray except those whom I guide, so seek My guidance and I shall guide you…" and so on. Q: Does it carry the same ruling as the Qurʾān? A: No. It is the speech of Allah, but it does not carry the ruling of the Qurʾān.