All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Perhaps the speaker meant what happened between the Prophet, sallallaahu ‘alayhi wa sallam, and the polytheists. It was narrated on the authority of Ubayy ibn Ka’b that the polytheists said to the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, "Tell us the lineage of your Lord." In response, Allaah The Almighty revealed (what means): {Say, "He Is Allaah, [Who Is] One, Allaah, the Eternal Refuge. He neither Begets nor Is Born. Nor is there to Him any equivalent.}[Quran 112:1-4][Ahmad and At-Tirmithi]
This Hadeeth shows that there is an origin for the story, but the polytheists, not a Christian man, were the ones who asked the Prophet, sallallaahu ‘alayhi wa sallam. This is supported by the fact that Surah Al-Ikhlaas is a Makkan Surah which was revealed before Hijrah, and that the Prophet, sallallaahu ‘alayhi wa sallam, first met the People of the Book in Madeenah.
Allaah Knows best.