The Quran

أَكانَ لِلنّاسِ عَجَبًا أَن أَوحَينا إِلىٰ رَجُلٍ مِنهُم أَن أَنذِرِ النّاسَ وَبَشِّرِ الَّذينَ آمَنوا أَنَّ لَهُم قَدَمَ صِدقٍ عِندَ رَبِّهِم ۗ قالَ الكافِرونَ إِنَّ هٰذا لَساحِرٌ مُبينٌ

Muhammad Asad

Do people deem it strange that We should have inspired a man from their own midst [with this Our message]:¹ "Warn all mankind, and give unto those who have attained to faith the glad tiding that in their Sustainer's sight they surpass all others in that they are completely sincere -² [Only] they who deny the truth say, "Behold, he is clearly but a spellbinder!"³

This connects with the end of the preceding Sarah, and particularly with the sentence, "There has come unto you [O mankind] an Apostle from among yourselves" (9:128; see also note 2 on 50:2).
Lit., "they have precedence (qadam) of truthfulness (sidq)": the latter term denoting a concord between what a person actually conceives in his mind or feels and what he expresses by word, deed or attitude - in other words, complete sincerity.
Lit., "He is indeed an obvious enchanter (sahir)" - thus implying that the "man from among yourselves" (i.e., Muhammad) did not really receive any revelation from God, but merely deluded his followers by means of his spellbinding eloquence (which is the meaning of sihr in this context): an accusation levelled by unbelievers of all times not merely against Muhammad but - as the Qur'an frequently states - against most of the earlier prophets as well. The term "those who deny the truth" refers, in this context, specifically to people who a priori reject the notion of divine revelation and, thus, of prophethood.