首楞嚴義疏注經 1.90

1.90

教主既非生滅心行說實相法，能聽之眾亦無一法可為領受。稱實說聽無有垢染，故云清眾。斯為究竟無上法會，故未曾有。

Since the Lord of the Teaching spoke the Dharma of the true mark with no arising-and-perishing mental activity, the assembly able to listen also had not a single dharma to be received. Speaking and listening in accord with reality were free from defilement, hence they are called the "pure assembly." This was the ultimate, unexcelled Dharma assembly, hence "unprecedented."