金光明最勝王經疏 2.12

2.12

問：何故釋迦現居此界，不自說壽，令他說耶？答：生宜聞故、彰四德故、除疑勝故。若唯自說，他非決故。

Question: Why did Śākyamuni, presently residing in this realm, not speak of his life-span himself, but let others speak of it? Answer: Because it was suitable for beings to hear, to display the four virtues, and because it was superior for removing doubts. If he had only spoken of it himself, others would not have been decisive.