UNSC Resolution 1441 is the one Iraq would have been in violation of in this case. It was passed November 8, 2002. Here is a link to the resolution:

UNSC Resolution 1441

This resolution was for the purpose of establishing and maintaining a UN inspections process in Iraq. As of the time that the US decided to invade Iraq, the UN inspectors had stated that as far as they were concerned, Saddam was cooperating with the inspections process to their satisfaction, and they wanted to continue with the process because they determined that it was working. Under the terms of the resolution, it was a determination they had the authority to make. Under the terms of the resolution, the US did not have the authority to disrupt the inspections process, which is exactly what the US did when it kicked the UN inspectors out of Iraq and invaded that country.

I will look for documentation of the UN's position as I have described it above tomorrow, and I will post it when I have it.