Seal of identity

"And in a hollow voice he spake, and said --
'Sohrab, that were a proof that could not lie.
If thou show this, then thou art Rustum's son.'

Then, with weak hasty fingers, Sohrab loosed
His belt, and near the shoulder bared his arm,
And show'd a sign in faint vermilion points
Prick'd: as a cunning workman, in Pekin,
Pricks with vermilion some clear porcelain vase,
An emperor's gift --at early morn he paints,
And all day long, and, when night comes, the lamp
Lights up his studious forehead and thin hands: --
So delicately prick'd the sign appear'd
On Sohrab's arm, the sign of Rustum's seal."

Sohrab and Rustum  Matthew Arnold