in reserve
Stored and set aside for possible (future) use.
Synonyms: reserve, spare, backup
Never reveal all of yourself to other people; hold back something in reserve so that people are never quite sure if they really know you. Michael Korda
So the first step out of childhood is made all at once, without looking before or behind, without caution, and nothing held in reserve. Ursula K. Le Guin
Not all who hesitate are lost. The psyche has many secrets in reserve. And these are not disclosed unless required. Joseph Campbell
The mule long keeps a kick in reserve for its master. French Proverb
An old man always keeps something in reserve. Swahili Proverb
The preacher must be like a chicken who always has an egg in reserve. German Proverb