وَأَنْ لَا يُشَمَّ رَائِحَتُهُ لَفْظًا
The wording should not openly disclose the smell of simile.
A metaphor typically presents the subject as though it belongs to the kind represented by the borrowed image. If an explicit comparison is added at the same point, that strong rhetorical claim can be weakened. “Zayd is a lion” and “Zayd is like a lion” are closely related in sense but not identical in rhetorical force.
The condition concerns overt wording that exposes the simile, not the absence of resemblance in thought. Resemblance remains the underlying basis of metaphor.