Challenge to prophethood

قَالَ فَأْتِ بِهِۦٓ إِن كُنتَ مِنَ ٱلصَّـٰدِقِينَ

[Pharaoh] said, "Then bring it, if you should be of the truthful."

وَيَقُولُونَ مَتَىٰ هَـٰذَا ٱلْوَعْدُ إِن كُنتُمْ صَـٰدِقِينَ

And they say, "When is [the fulfillment of] this promise, if you should be truthful?"