The Quran

قالَت لَهُم رُسُلُهُم إِن نَحنُ إِلّا بَشَرٌ مِثلُكُم وَلٰكِنَّ اللَّهَ يَمُنُّ عَلىٰ مَن يَشاءُ مِن عِبادِهِ ۖ وَما كانَ لَنا أَن نَأتِيَكُم بِسُلطانٍ إِلّا بِإِذنِ اللَّهِ ۚ وَعَلَى اللَّهِ فَليَتَوَكَّلِ المُؤمِنونَ

Muhammad Asad

Their apostles answered them: "True, we are nothing but mortal men like yourselves: but God bestows His favour upon whomever He wills of His servants. Withal, it is not within our power to bring you a proof [of our mission], unless it be by God's leave - and [so] it is in God that all believers must place their trust.¹

I.e., it is to the contents of the divine message propounded to them that all seekers after truth must turn for illumination (see 7:75 and 13:43, as well as the corresponding notes). The Qur'an dwells in many places (e.g., in 6:109-111 or 13:31) on the futility - moral as well as intellectual - of the demand that the divine origin of a prophetic message should be proved by tangible, extraneous means: for, a morally valid and intellectually justifiable conviction of the intrinsic truth of such a message can be gained only through "conscious insight accessible to reason" (12:108).