Observation billing

Ok, I need a definitive answer to coding observation. I know only the provider who admitted pt to OBS can bill OBS codes. What my coworkers are arguing about is billing by a different provider when they see one of their own pt's in hosp OBS setting. I have been told that we are to bill Medicare and MDCR replacement plans with 99224-99226 when the pt is an est'd patient of the provider's group. PLEASE show me where this rule is written in the CMS manual. The chpt 12 I have from CMS is effective 11-28-11 and there it instructs us to code est'd pt office visit codes for OBS status
Thanks, Lisa