Is the Hadith in which the Prophet (peace be upon him) forbade Ali from wearing a ring on the pinky and thumb finger authentic, and what is the ruling on the Prophet wearing a ring on the pinky finger despite this prohibition?

The Hadith (definition) of Ali ibn Abi Talib (may Allah be pleased with him): "The Messenger of Allah (peace be upon him) forbade me from wearing a ring on this [finger] and on this [finger]," meaning the little finger and the thumb. This was narrated by Ibn Majah with a chain of narrators whose men are trustworthy, but the word "little finger" (al-khinsir) in it is anomalous (shadhdh).

Anomalous (shadhdh) refers to what a trustworthy narrator relates that contradicts the narration of more numerous or more حافظ (reliable in memory) trustworthy narrators. A group of trustworthy narrators related it with a different wording, such as the narrations of Imam Muslim, Abu Dawud, Al-Tirmidhi, and Al-Nasa'i, where it is mentioned that the Prophet (peace be upon him) pointed to the index finger and the middle finger, and that he forbade wearing rings on them.

This is supported by what Al-Bukhari and Muslim narrated, that the Prophet (peace be upon him) used to wear his ring on his little finger, and Muslims have unanimously agreed that the Sunnah (definition) is for a man's ring to be on his little finger.