The Dhammika Sutta is part of the Sutta Nipata(Sn 2.14). In this sutta, the Buddha instructs a lay disciple named Dhammika on rules for monks and on the "layman's rule of conduct" (gahatthavatta).
Read more about Dhammika Sutta: Dhammika Asks of Virtue, Monastic Virtue, Lay Virtue