Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(And they will say) i.e. the disbelievers of Mecca: (If only a portent) a sign (were sent down upon him) i.e. to Muhammad (pbuh) (from his Lord!) to vouchsafe his claim. (Then say) O Muhammad: (The Unseen) regarding the sending of such a sign (belongeth to Allah. So wait) for my destruction! (Lo, I am waiting with you) for your destruction.
And they, the people of Mecca, say, ‘Why has a sign not been sent down on him, on Muhammad (s), from his Lord?’, as was the case with [previous] prophets, in the way of a she-camel, a staff or a [glowing] hand. Then say, to them: ‘The Unseen, that which is concealed from servants, in other words, its affair, belongs only to God, and it is from this [Unseen] that signs come forth; therefore He alone can bring them forth: mine is only to covey the Message. So wait, for the chastisement, if you do not believe. I am waiting with you’.
These stubborn, lying disbelievers said, "Why would not a sign be revealed to Muhammad from his Lord.'' They meant a sign such as given to Salih. Allah sent the she-camel to Thamud. They wanted Allah to change the mount of As-Safa into gold or remove the mountains of Makkah and replace them with gardens and rivers. Allah is capable of doing all of that, but He is All-Wise in His actions and statements. Allah said:
(Blessed be He Who, if He wills, will assign you better than (all) that -- Gardens under which rivers flow (Paradise) and will assign you palaces (in Paradise). Nay, they deny the Hour, and for those who deny the Hour, We have prepared a flaming Fire.)25:10-11 He also said:
(And nothing stops Us from sending the Ayat but that the people of old denied them.) 17:59 Allah's way of dealing with His creatures is that He would give to them if they asked things from Him. But if they then didn't believe He would expedite punishment for them. When Allah's Messenger was given the choice of Allah giving the people what they requested but if they didn't believe they would be punished, or that their request would not be answered immediately, Allah's Messenger chose the latter. Allah guided His Prophet to answer their question by saying:
فَقُلْ إِنَّمَا الْغَيْبُ للَّهِ
(Say: "The Unseen belongs only to Allah...'') This Ayah means that the matter in its entirety is for Allah. He is well aware of the outcome of all matters.
("...so wait you, verily, I am with you among those who wait.'') If you would not believe unless you witness that which you asked for, then wait for Allah's judgement for me, as well as for yourselves. Nonetheless, they had witnessed some of the signs and miracles of the Prophet, which were even greater than what they had asked for. In their presence, the Prophet pointed to the moon when it was full and it split into two parts, one part behind the mountain and the other before them. If they were seeking the guidance and firm knowledge by asking for signs, Allah would have known that and would have granted them what had been requested. But Allah knew that it was their obstinacy that was behind their request. Therefore Allah left them to suffer in their suspicion and doubt. Allah knew that none of them would believe. This is similar to Allah's statements:
(And even if We had sent down unto them angels, and the dead had spoken unto them, and We had gathered together all things before their very eyes, they would not have believed, unless Allah willed.)6:111 This was in addition to their arrogance. As Allah said in another Ayah:
(And even if We had sent down unto you (O Muhammad) a Message written on paper so that they could touch it with their hands, the disbelievers would have said: `This is nothing but obvious magic!')6:7 Such people don't deserve to have their requests answered, for there is no benefit in answering them. These people are obstinate and stubborn as a result of their corruption and immorality. Therefore Allah told His Messenger to say:
(So wait you, verily, I am with you among those who wait.)
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
That is, "Though they demanded a sign as a proof of the fact that he was a true Prophet and was presenting true Revelations, this was not based on a genuine desire for recognizing and accepting the Truth but was merely a pretext for rejecting it. For they had no intention of molding their habits, their morality, their social system, in short, reforming every aspect of their life in accordance with the implications of the acceptance of the Message of the Prophet. As they did not intend to believe in it, they would say, even after seeing a Sign, that no Sign had been shown to them. Obviously, they would not give up that "freedom" which they were enjoying in following their desires and lusts, by accepting the Unseen (the creeds of Tauhid and the Hereafter), which would bind them to the absolute moral principles."
That is, "I have presented before you what Allah has sent down to me; but what He has not sent down to me, is "unseen" both for me and for you, and it is entirely with Allah to reveal or not to reveal it. Now if you will not believe unless Allah sends down what He has not sent down, you may wait for it: I, too, shall wait and sec whether your persistent demand is or is not satisfied".