وَاِذَا لَمْ تَأْتِهِمْ بِاٰيَةٍ قَالُوْا لَوْلَا اجْتَبَيْتَهَاۗ قُلْ اِنَّمَآ اَتَّبِعُ مَا يُوْحٰٓى اِلَيَّ مِنْ رَّبِّيْۗ هٰذَا بَصَاۤىِٕرُ مِنْ رَّبِّكُمْ وَهُدًى وَّرَحْمَةٌ لِّقَوْمٍ يُّؤْمِنُوْنَ ( الأعراف: ٢٠٣ )
Wa izaa lam taatihim bi aayatin qaaloo law lajtabai tahaa; qul innamaaa attabi'u maa yoohaaa ilaiya mir Rabbee; haazaa basaaa'iru mir Rabbikum wa hudanw wa rahmatul liqawminy yu'minoon (al-ʾAʿrāf 7:203)
And when you, [O Muhammad], do not bring them a sign [i.e., miracle], they say, "Why have you not contrived it?" Say, "I only follow what is revealed to me from my Lord. This [Quran] is enlightenment from your Lord and guidance and mercy for a people who believe." (Al-A'raf  : 203)
1 Mufti Taqi Usmani
When you do not bring them a sign, they say, “Could you not make up one?”Say, “I only follow what is revealed to me from my Lord.” This is (a Book of) insights from your Lord and a guidance and mercy for a people who believe.