Joe, et al., the IMDB page for Lilies of the Field says:
Amen
(uncredited)
by Jester Hairston
sung by Sidney Poitier
Dubbed by Jester Hairston

Now, THERE's a character, and a legend
in the choral music business for his spirituals arrangements --
he almost, but not quite, lived to one hundred years.

Nice to know that is HIS singing voice,
seeing as this was not in the credits.