17 And a stone was brought and laid upon the mouth of the den; and the king sealed it with his own signet, and with the signet of his lords; that the purpose might not be changed concerning Daniel.
Certainly it had been decided by Darius to save Daniel from the present danger with any possible means; therefore he sealed the entrance of the den with his own signet and invited the satraps to do the same. This had to be done for two reasons: first, in order that [the satraps] might not steal there without being seen and, after rolling off the lid, might kill him; second, in order that they might not spread among the people the rumor that some fraud had been used inside the den.
Verse 17. "A single stone was brought and placed over the opening of the pit, and the king sealed it with his ring..." He sealed with his ring the rock by which the opening of the pit was shut up, so that the enemies of Daniel might not make any attempt to harm him. For he had entrusted him to the power of God, and although not worried about lions, he was fearful of men. He also sealed it with the ring of his nobles, in order to avoid all ground for suspicion so far as they were concerned.
[AD 373] Ephrem the Syrian on Daniel 6:17