When I am in Rome, I fast as the Romans do; when I am at Milan, I do not fast. So likewise you, whatever church you come to, observe the custom of the place.

[Cum Romanum venio, ieiuno Sabbato; cum hic sum, non ieiuno: sic etiam tu, ad quam forte ecclesiam veneris, eius morem serva, si cuiquam non vis esse scandalum nec quemquam tibi.]

St. Ambrose (339-397) Roman prelate, Bishop of Milan [Aurelius Ambrosius]
In Augustine, Epistulae, Letter 36 (c. AD 400)

Alt trans.:

• Popularly, "When in Rome, do as the Romans do."
• "When I am at Rome, I fast on a Saturday; when I am at Milan, I do not. Follow the custom of the church where you are."
• "When I am here, I do not fast on the Sabbath; when I am in Rome, I fast on the Sabbath."
• Alternately given as "If you are at Rome, live in the Roman style; if you are elsewhere, live as they live there. [Si fueris Romae, Romano vivito more; / Si fueris alibi, vivito sicut ibi.]" in J. Taylor, Ductor Dubitantium, or the Rule of Conscience, I.i.5 (1660).

Various Augustine citations described:

• Epistulae 36, 14 or 32
• Letter 54 to Januarius
• Epistle to Januarius, 2, sec. 18
• Epistle to Casualanus, 36, sec. 32