

I think the passage you're looking for is the following.

Matthew 5:17    "Think not that I have come to abolish the law and the 
prophets; I have come not to abolish them but to fulfil them. 
Matthew 5:18   For truly, I say to you, till heaven and earth pass away, 
not an iota, not a dot, will pass from the law until all is accomplished. 
Matthew 5:19   Whoever then relaxes one of the least of these commandments 
and teaches men so, shall be called least in the kingdom of heaven; but he 
who does them and teaches them shall be called great in the kingdom of 
heaven. 
Matthew 5:20   For I tell you, unless your righteousness exceeds that of 
the scribes and Pharisees, you will never enter the kingdom of heaven. 

There are several problems with this.  The most serious is that the
Law was regarded by Jews at the time (and now) as binding on Jews, but
not on Gentiles.  There are rules that were binding on all human
beings (the so-called Noachic laws), but they are quite minimal.  The
issue that the Church had to face after Jesus' death was what to do
about Gentiles who wanted to follow Christ.  The decision not to
impose the Law on them didn't say that the Law was abolished.  It
simply acknowledged that fact that it didn't apply to Gentiles.  This
is a simple answer, which I think just about everyone can agree to.
(A discussion of the issue in more or less these terms is recorded
in Acts 15.)

However there's more involved.  In order to get a full picture of the
role of the Law, we have to come to grips with Paul's apparent
rejection of the Law, and how that relates to Jesus' commendation of
the Law.  At least as I read Paul, he says that the Law serves a
purpose that has been in a certain sense superceded.  Again, this
issue isn't one of the abolition of the Law.  In the middle of his
discussion, Paul notes that he might be understood this way, and
assures us that that's not what he intends to say.  Rather, he sees
the Law as primarily being present to convict people of their
sinfulness.  But ultimately it's an impossible standard, and one that
has been superceded by Christ.  Paul's comments are not the world's
clearest here, and not everyone agrees with my reading.  But the
interesting thing to notice is that even this radical position does
not entail an abolition of the Law.  It still remains as an
uncompromising standard, from which not an iota or dot may be removed.
For its purpose of convicting of sin, it's important that it not be
relaxed.  However for Christians, it's not the end -- ultimately we
live in faith, not Law.

While the theoretical categories they use are rather different, in the
end I think Jesus and Paul come to a rather similar conclusion.  The
quoted passage from Mat 5 should be taken in the context of the rest
of the Sermon on the Mount, where Jesus shows us how he interprets the
Law.  The "not an iota or dot" would suggest a rather literal reading,
but in fact that's not Jesus' approach.  Jesus' interpretations
emphasize the intent of the Law, and stay away from the ceremonial
details.  Indeed he is well known for taking a rather free attitude
towards the Sabbath and kosher laws.  Some scholars claim that Mat
5:17-20 needs to be taken in the context of 1st Cent. Jewish
discussions.  Jesus accuses his opponents of caring about giving a
tenth of even the most minor herbs, but neglecting the things that
really matter: justice, mercy and faith, and caring about how cups and
plates are cleaned, but not about the fact that inside the people who
use them are full of extortion and rapacity.  (Mat 23:23-25) This, and
the discussion later in Mat 5, suggest that Jesus has a very specific
view of the Law in mind, and that when he talks about maintaining the
Law in its full strength, he is thinking of these aspects of it.
Paul's conclusion is similar.  While he talks about the Law being
superceded, all of the specific examples he gives involve the
"ceremonial law", such as circumcision and the Sabbath.  He is quite
concerned about maintaining moral standards.

The net result of this is that when Paul talks about the Law being
superceded, and Jesus talks about the Law being maintained, I believe
they are talking about different aspects of the Law.  Paul is
embroiled in arguments about circumcision.  As is natural in letters
responding to specific situations, he's looking at the aspect of the
Law that is currently causing trouble: the Law as specifically Jewish
ceremonies.  He certainly does not intend to abolish divine standards
of conduct.  On the other hand, when Jesus commends the Law, he seems
to be talking the Law in its broadest implications for morals and
human relationships, and deemphasizing those aspects that were later
to give Paul so much trouble.