The Cattle - 6:37
"They say: "Should not a sign (miracle) have descended on him from your Lord!"Say: "Allah can send down a sign whenever He wills, but most of them do not know it."
Allah can send down a verse at any time:
The Poets/26:4
And if We willed, We could send down upon them a sign from the sky, and they would submit to it.
Prophet Jonah/10:99
Had your Lord willed, all those on the earth would have believed. Then will you force the people to believe?