"And if their turning away" from you, and their denial of you, "is difficult for you, then if you are able to seek a tunnel into the earth," meaning: a passage, that you may enter into it, "or a staircase into the sky," meaning: to the sky, that you may ascend it, "and bring them a sign." This was when they asked for a sign.

Muhammad said: The meaning is: if you are able to do this, then do it; he omitted "then do it" since there was that in the speech which indicated it.