《太平广记·卷二十七·神仙二十七》
他十五六岁的时候，忽然不知去哪里了，大概是周游天下寻师访道去了。
十五六岁，忽不知所之，盖游天下寻师访道矣。
When he was fifteen or sixteen, he suddenly disappeared without a trace, probably to travel around the world in search of a teacher and the Way.