فَاستَفتِهِم أَهُم أَشَدُّ خَلقًا أَم مَن خَلَقنا ۚ إِنّا خَلَقناهُم مِن طينٍ لازِبٍ

Then inquire of them, [O Muhammad], "Are they a stronger [or more difficult] creation or those [others] We have created?" Indeed, We created men from sticky clay.