Athar of Umar about employing non-muslims

It is narrated: >Imaam Ahmad reported that Abu Moosa al-Ash’ari (may Allaah be pleased with him) said: “I said to ‘Umar (may Allaah be pleased with him). ‘I have a Christian scribe.’ He said, ‘What is wrong with you, may Allaah strike you dead! Have you not heard the words of Allaah (interpretation of the meaning), “O you who believe! Take not the Jews and the Christians as awliya’ (friends, protectors, helpers, etc.), they are but awliya’ to one another…” [al-Maa’idah 5:51]”? Why do you not employ a haneef [i.e., a Muslim]?’ I said, ‘O Ameer al-Mu’mineen, I benefit from his work and he keeps his religion to himself.’ He said, ‘I will never honour them when Allaah has humiliated them, and I will never bring them close to me when Allaah has expelled them from His mercy.’” What is the authenticity of this narration? Likewise, if it is authentic, isn't Umar RA being a bit harsh? He simply employed him as a scribe nothing else.