There is a story of a man who went to Abu Bakr for judgement after going and receiving judgement from the Prophet, sallallaahu ‘alayhi wa sallam, and Abu Bakr turns him away, so he proceeds to ‘Umar, who, upon hearing that he has already received a judgement of the Prophet, sallallaahu ‘alayhi wa sallam, kills him. Is this story authentic?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

This report was cited by Az-Zaylaʻi in his revision of the ahaadeeth narrated in Al-Kashshaaf. He underlined that the report was narrated on the authority of Ibn Abi Haatim and Ibn Marduwayh. Az-Zaylaʻi classified it as weak because it is Mursal (i.e. a Hadeeth narrated by a Tabi'i [successor of the Prophet's Companions]) with an unconnected chain of narration) and its narrators include Ibn Laheeʻah, who was declared a weak narrator.

Ibn Katheer said, "It was also cited by Ibn Marduwayh on the authority of Ibn Laheeʻah from Abu Al-Aswad. It is a Ghareeb report (hadeeth reported by one reliable or unreliable narrator and which differs in context with another one reported by a group of reliable narrators), and it is Mursal; and Ibn Laheeʻah was classified as a weak reporter."

Allah knows best.