Imam al-Shafi'i (may Allah have mercy on him) said: I said to some people: "The Sunnah of the Messenger (peace and blessings be upon him) is that the hand be cut off for a quarter of a dinar or more. So how can you say that the hand is not to be cut off except for ten dirhams or more? What is your proof for this?" He said: "We have narrated from Sharik, from Mansur, from Mujahid, from Ayman, from the Prophet (peace and blessings be upon him), something resembling our view." I said: "Do you know who Ayman is? The Ayman from whom Ata narrates was a young man, perhaps younger than Ata, and Ata narrated from him a hadith from Tabi', the son of Ka'b's wife, from Ka'b. So this is munqati' (disconnected), and a munqati' hadith is not a proof." He said: "Sharik bin Abdullah narrated it from Mujahid, from Ayman bin Umm Ayman, the maternal brother of Usamah." I said: "You have no knowledge of our companions. Ayman, the brother of Usamah, was killed alongside the Messenger of Allah (peace and blessings be upon him) on the day of Hunayn, before Mujahid was born, and he did not live on after the Prophet (peace and blessings be upon him) so as to narrate from him."
The Shaykh (al-Bayhaqi) (may Allah have mercy on him) said: The narration of Ata from Ayman to which al-Shafi'i alluded is a different hadith from this one.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السرقة / 17175