His saying, exalted and majestic is He:

﴿And they said, 'Why has no sign been sent down to him from his Lord?' Say, 'The signs are only with Allah, and I am only a clear warner.'﴾ ﴿Is it not sufficient for them that We have sent down to you the Book which is recited to them? Indeed, in that is a mercy and a reminder for a people who believe.﴾ ﴿Say, 'Sufficient is Allah as a witness between me and you. He knows what is in the heavens and the earth. And those who have believed in falsehood and disbelieved in Allah, it is they who are the losers.'﴾

The pronoun in 'they said' refers to Quraysh and some of the Jews; because they were teaching Quraysh such arguments: he has not brought you anything like what Moses brought with the staff and other things. Ibn Kathir, Hamzah, Al-Kisai, and Abu Bakr narrated from Asim: 'A sign from his Lord,' and Nafi, Ibn Amir, Abu Amr, and Hafs narrated from Asim: 'Signs.' So Allah, exalted and majestic is He, commanded His Prophet, blessings and peace be upon him, to teach them that this matter is in the hand of Allah, blessed and exalted is He, and that it is not brought down by suggestion and wishing, and that he was sent as a warner, and was not commanded with anything other than that. In the Mushaf of Ubayy: 'Why does he not bring us signs from his Lord? Say, 'The signs are only with Allah.'

Then he argued against them in their request for a sign by the matter of the Qur'an, which is the greatest of signs and a miracle for both jinn and humans. He said: ﴿Is it not sufficient for them that We have sent down to you the Book?﴾ Then he affirmed what is in it of mercy and a reminder for the believers. His saying: ﴿Is it not sufficient for them﴾ is an answer to the one who said: ﴿Why has no sign been sent down?﴾

Al-Tabari narrated that this verse was revealed because 'a group of believers came to the Prophet, blessings and peace be upon him, with books they had written in which was some of what the Jews had told them about something from the Torah. The Messenger of Allah, blessings and peace be upon him, denied that, saying: 'This is sufficient misguidance, a people who turned away from what their Prophet brought them to what another brought.' And the verse was revealed because of that.

Qadi Abu Muhammad, may Allah have mercy on him, said:

The first interpretation aligns with the context of the verses.

Then Allah, exalted and majestic is He, commanded His Prophet, blessings and peace be upon him, to rely on the matter of Allah, blessed and exalted is He, and to make Him sufficient as a witness and a judge between him and them by His knowledge and His encompassing all their affairs. And His saying: 'in falsehood' means: in idols and false deities and what follows their command from beliefs. Falsehood is when an action is done intending a matter, and that matter does not come from that action. The idols are intended by their most complete and most successful in the claim of their worshippers, and the most complete and most successful is only to reject them; thus they are false. The rest of the verse is clear.