Question regarding I-485, please help! Thanks!

I was told by one law firm that a person can\'t file I-485 unless he/she has enterred and stayed in U.S. for three months.

I checked USCIS.org and other resource, but couldn’t find anything related to this rule. Is this true, if not, are there any similar rule? Can anybody who knows this particular rule provide me some comments?

Thanks a lot!