摩訶止觀 2.18

2.18

口說默者，九十日，身常行，無休息；九十日，口常唱阿彌陀佛名，無休息；九十日，心常念阿彌陀佛，無休息。或唱念俱運，或先念後唱，或先唱後念，唱念相繼，無休息時。若唱彌陀，即是唱十方佛功德等，但專以彌陀為法門主。舉要言之，步步、聲聲、念念，唯在阿彌陀佛。

Regarding vocal speech and silence: for ninety days, the body constantly walks without resting; for ninety days, the mouth constantly chants the name of Amitābha Buddha without resting; for ninety days, the mind constantly contemplates Amitābha Buddha without resting. Either chant and contemplate simultaneously, or contemplate first and chant later, or chant first and contemplate later, chanting and contemplating in succession with no time of rest. If one chants Amitābha, that is chanting the merits of the Buddhas of the ten directions and so forth, yet one exclusively takes Amitābha as the master of the Dharma gate. To sum up the essentials: step by step, sound by sound, thought-moment by thought-moment, be solely on Amitābha Buddha.