"Do you have any authority for this?" meaning: you have no proof for what you have said. "Do you say about Allah what you do not know?" meaning: Yes, you have indeed said about Allah what you do not know.
﴿إِنْ عِنْدَكُمْ مِنْ سُلْطَانٍ بِهَذَا﴾ أَيْ: مَا عِنْدَكُمْ مِنْ حُجَّةٍ بِهَذَا الَّذِي قُلْتُمْ ﴿أَتَقُولُونَ عَلَى الله مَا لَا تعلمُونَ﴾ أَيْ: نَعَمْ، قَدْ قُلْتُمْ عَلَى اللَّهِ مَا لَا تَعْلَمُونَ
