大比丘三千威儀 1.26

1.26

復有四事應行：一者塔使；二者比丘僧使；三者三師使；四者學盡三師所知，應從師求盡至明者。

There are also four circumstances under which one should depart: first, on a mission for the stūpa; second, on a mission for the bhikṣusaṅgha; third, on a mission for the three masters; fourth, having learned everything known by the three masters, one should seek from a master to exhaustively reach those who are clear-sighted.