首楞嚴義疏注經 4.475

4.475

此是如來知時知機，令自選擇，樂欲相應起隨順行。如下文云「我今欲令阿難開悟，二十五行誰當其根？兼我滅後，此界眾生入菩薩乘，求無上道，何方便門得易成就？」故云日劫相倍也。

This is the Tathāgata knowing the time and knowing the faculties, causing one to choose for oneself, so that with an inclination that accords, one may initiate conforming practice. As a later passage says: "I now wish to cause Ānanda to be enlightened: among the twenty-five practices, which accords with his faculty? And after my passing, for sentient beings in this world entering the Bodhisattva Vehicle to seek the unexcelled path, through which expedient gate can it be easily accomplished?" Therefore it says "a day and a kalpa will be doubled".