http://www.accesstoinsight.org/tipitaka ... .than.html
When this was said, Saccaka the Nigaṇṭha-son was silent a second time.
Then the Blessed One said to him, "Answer now, Aggivessana. This is not the time to be silent. When anyone doesn't answer when asked a legitimate question by the Tathāgata up to three times, his head splits into seven pieces right here."
Now on that occasion the spirit (yakkha) Vajirapāṇin [Thunderbolt-in-Hand], carrying an iron thunderbolt, was poised in the air above Saccaka the Nigaṇṭha-son, (thinking,) "If Saccaka the Nigaṇṭha-son doesn't answer when asked a legitimate question by the Blessed One up to three times, I will split his head into seven pieces right here."
The Blessed One saw the spirit Vajirapāṇin, as did Saccaka the Nigaṇṭha-son. So Saccaka — afraid, terrified, his hair standing on end — seeking shelter in the Blessed One, seeking a cave/asylum in the Blessed One, seeking refuge in the Blessed One — said to the Blessed One, "Let Master Gotama ask me. I will answer.
It's surprising that the Buddha didn't scold Vajirapāṇin; rather he could be seen as an accomplice to the spirit's threats. I would be most appreciative for any insight on how best to interpret the above.