Ebrahim AS

I was telling a christain person about Islam and he told me that Ebrahim (AS) was a sinner cause he commited zina with Hajira (AS) and Ismail (AS) was born.Therefore the prodigy of Ismail (AS) is illegitimate. How do I tell him that Sarah (AS) gave Ebrahim
(AS) the go ahead to marry Hajirah (AS) and have a child from her as she could not bear children?
Is it possible for Mufti-Saheb to qoute from Quraan+Hadith?