Musannaf Ibn Abi Shayba 26:343

Muhammad ibn Abi ‘Adi narrated to us, from Ibn ‘Awn, who said: It was mentioned to Muhammad [ibn Sirin] that ‘Umar ibn ‘Abd al-‘Aziz wrote regarding the *humala* (those of unknown lineage/adopted): They shall not inherit except by the testimony of witnesses. He said: So Muhammad said: The Emigrants (*Muhajirun*) and the Helpers (*Ansar*) inherited by their lineage which was [established] during the Pre-Islamic Era (*Jahiliyyah*), and I deny that ‘Umar wrote this.