妙法蓮華經玄義 6.139

6.139

無問妙者，如文云：「無問而自說，稱歎所行道，從三昧安詳而起，告舍利弗，說佛智慧。」又「宿世因緣，吾今當說」，即是無問妙也。

The subtlety of udāna (unprompted utterances): as the text says, 'He expounded spontaneously without being asked, praising the path he had practiced; calmly arising from samādhi, he addressed Śāriputra, expounding the wisdom of the buddhas.' Also: 'The causes and conditions of past lives, I shall now expound.' This is the subtlety of udāna.