Reference / Source: Ibn Sa'd narrated it in "al-Tabaqat al-Kubra" (4/89 and 94) from his teacher Muhammad bin 'Umar al-Waqidi with this same chain of his.
Criticism and praise of narrators: Abd al-Hakim bin Abd Allah bin Abi Farwah is "la ba'sa bihi" (acceptable), but his teacher "Abd Allah bin 'Amr bin Sa'id bin al-'As" is majhul (unknown) and not recognised. He is not the son of the subject of this biography (the Companion being discussed), as might immediately come to mind, because the subject of the biography ('Amr bin Sa'id) had no descendants ('aqib), as Ibn Sa'd and others stated explicitly. He may be one of the grandsons of one of his brothers.

Details of the narration: Muhammad bin 'Umar al-Waqidi narrated the story of Abu Uhayhah Sa'id bin al-'As bin Umayyah with his son Khalid bin Sa'id (when he embraced Islam) in greater detail than here, through another chain, which will come later with the author at number (5160).

Important point: The report of the conversion to Islam of Khalid bin Sa'id bin al-'As and his brother 'Amr, and their emigration to Abyssinia, is "well known" among the scholars of the Maghazi and the Sirah.

Reference / Source: Ibn Sa'd (4/90) narrated the story of 'Amr bin Sa'id's conversion to Islam and his emigration to Abyssinia with his wife Fatimah bint Safwan from Muhammad bin 'Umar al-Waqidi, from Ja'far bin Muhammad bin Khalid bin al-Zubayr bin al-'Awwam, from Muhammad bin Abd Allah bin 'Uthman bin 'Affan, in "mu'dal" form (with two or more consecutive narrators missing from the chain).