The brief prayer to be reborn in Amitabha's pure land of  Sukhavati  (Tibetan Dewachen) is mentioned in the Chenrezig prayer. The reason Sukhavati is important is that most of the pure lands that exist do not allow individuals who have not attained the higher bodhisattva's levels, but Amitabha in his infinite compassion said he would allow ordinary persons who were extremely devoted to enter Sukavati where they could practice the definitive Dharma continuously.

This PDF is a 16 page detailed description on the Brief Prayer for Rebirth in Sukhavati.