Concept:
Poisons are dangerous substances and their sale is controlled by law.
Step 1: Need for record.
Because poisons can cause serious harm, their sale must be documented properly.
Step 2: Poison register.
The details of poison sale, purchaser, quantity, and purpose are recorded in a poison register.
Step 3: Why this is required.
This helps prevent misuse and allows legal tracking of poison sales.
Step 4: Final answer.
Thus, poison sale requires entry in poison register.
\[
\boxed{\text{Entry in poison register}}
\]