Pratyutpanna Samādhi Sūtra

Bodhisattvas hear about the Buddha Amitābha and call him to mind again and again in this land. Because of this calling to mind, they see the Buddha Amitābha. Having seen him they ask him what dharmas it takes to be born in the realm of the Buddha Amitābha. Then the Buddha Amitābha says to these bodhisattvas: "If you wish to come and be born in my realm, you must always call me to mind again and again, you must always keep this thought in mind without letting up, and thus you will succeed in coming to be born in my realm.