«فَإِنْ كُنْتَ صَدَعْتَ عَنْ وَصْفِكَ بِالْيَقِينِ فَأْتِ بِآيَةٍ إِنْ كُنْتَ مِنَ الصَّادِقِينَ»
If your description was spoken with certainty, produce a clear proof of its truth.
Fa-in kunta ... faʾti bi-āyah makes the claimant’s self-description the condition and demands evidence as the response. In this literary gathering, āyah means a convincing sign or specimen of skill, not automatically a miracle.
The formula echoes Quranic diction, but its immediate pragmatic function is the testing of literary competence. The challenge does not deny that the speaker can talk; it asks him to perform at the level he has claimed.
Practice: What would answer the challenge? An actual demonstration, not further self-praise.