in (one's) possession
Owned, held by, or under the custody of someone. With her father's inheritance in her possession, Samantha intends to start her own business. The detective wanted to know how long the victim's diary had been in the suspect's possession. Are you on your way to get it, or is literally already in your possession?
*in someone's possession
held by someone; owned by someone. (*Typically: be ~; come [into] ~.) The book is now in my possession. How long has this object been in your possession?