觀無量壽佛經疏 2.7

2.7

又言證信者。欲明阿難稟承佛教傳持末代，為對眾生故如是觀法我從佛聞，證誠可信。故名證信序。此就阿難解也。

Furthermore, regarding "verifying faith": this clarifies that Ānanda, having received the Buddha's teaching, transmits and maintains it for latter ages. For the sake of sentient beings [he says]: "Contemplation dharmas such as this, I heard from the Buddha; they are authenticated and can be trusted." Hence it is called the preface verifying faith. This is explained with respect to Ānanda.