It's in every version of Revised Ranger, across the various Unearthed Arcana - basically every revision of ranger after the original PHB:
"If your animal companion is ever slain, the magical bond you share allows you to return it to life. With 8 hours of work and the expenditure of 25 gp worth of rare herbs and fine food, you call forth your companion’s spirit and use your magic to create a new body for it. You can return an animal companion to life in this manner even if you do not possess any part of its body.
If you use this ability to return a former animal companion to life while you have a current animal companion, your current companion leaves you and is replaced by the restored companion."
