The seventh sutta of the Majjhima Nikāya (M.i.36ff). The Buddha says that, even as a dirty piece of cloth takes dyes badly, so in an impure heart bliss is not to be found. He then proceeds to enumerate the heart's impurities and to show how they can be cleansed. Sundarika Bhāradvāja, who is present, asks the Buddha if he has bathed in the Bāhukā. The Buddha then gives a list of places whose waters are considered holy, and declares that the real cleansing is the cleansing of the heart; "to love all that lives, speak truth, slay not nor steal, no niggard be but dwell in faith." Bhāradvāja seeks ordination and becomes an arahant.
It is evidently this sutta which is referred to in the Sumangalavilāsinī (DA.i.50, 123), as the Vattha Sutta.