大比丘三千威儀 1.54

1.54

There are five matters where one should not pay homage: first, whether reciting sūtras or holding sūtras, one should not pay homage to a sthavira; second, when the sthavira is in a lower place and oneself is in a higher place, one should not pay homage; third, when the sthavira is in front or has already walked past, one should not pay homage from behind; fourth, one must not pay homage to a sthavira while remaining on one's seat; fifth, one must not pay homage to the Buddha while wearing a cap. If [done to] the three masters, the bhikṣusaṅgha, or a sthavira, the offense is grave.