«ثُمَّ أَخَذَ يُبْدِي مَا فِي وِطَابِهِ وَيُعْجِبَ الْحَاضِرِينَ بِفَضْلِ خِطَابِهِ»
Then he began displaying what he held in store and astonishing the audience with his outstanding speech.
Wiṭāb are skins or containers traditionally used for liquids such as milk. Here the image suggests his stored learning and eloquence, not a literal pouring out of contents.
Akhadha yubdī means that he began to reveal his gifts; the sustained speech changes the audience’s impression of him.