When thy next neighbours house is all on fire,
’Tis thy concern to make his flames expire;
For fire will gather strength if let alone,
And with thy neighbours house burn down thine owne.

[Nam tua res agitur, paries cum proximus ardet.
Et neglecta solent incendia sumere vires.]

Horace (65–8 BC) Roman poet, satirist, soldier, politician [Quintus Horatius Flaccus]

Epistles [Epistularum, Letters], Book 1, ep. 18 “To Lollius,” l. 84ff (1.18.84-85) (20 BC) [tr. A. B.; ed. Brome (1666)]
    (Source)

On the need to defend friends who are being slandered by others.

(Source (Latin)). Other translations:

The case is thyne, thy neighboures house when it doth flame up bright,
And burninges thowght but smal, or now have grown to dreedful might.
[tr. Drant (1567)]

For you're in danger when the Next's on fire,
And Flames neglected often blaze the higher.
[tr. Creech (1684)]

When flames your neighbour's dwelling seize,
Your own with instant rage shall blaze;
Then haste to stop the spreading fire,
Which, if neglected, rises higher.
[tr. Francis (1747)]

He that espies a neighbour's roof on fire
And calmly sees the flames to heaven aspire,
Will find them gather strength, till let alone
They with his neigbour's house burn down his own.
[tr. Howes (1845)]

For it is your own concern, when the adjoining wall is on fire: and flames neglected are wont to gain strength.
[tr. Smart/Buckley (1853)]

No time for sleeping with a fire next door;
Neglect such things, they only blaze the more.
[tr. Conington (1874)]

With the next house in flames, best look ahead —
A fire neglected's pretty sure to spread.
[tr. Martin (1881)]

For your own business is affected when your neighbor's wall is on fire, and flames neglected gather strength.
[tr. Dana/Dana (1911)]

'Tis your own safety that's at stake, when your neighbour's wall is in flames, and fires neglected are wont to gather strength.
[tr. Fairclough (Loeb) (1926)]

When flames your neighbor’s dwelling seize,
Your own with instant rage shall blaze;
Then haste to stop the spreading fire,
Which, if neglected, rises higher.
[tr. A. F. Murison (1931); ed. Kramer, Jr. (1936)]

When your neighbor's house catches fire, your place is threatened,
And flames that are disregarded usually burn brighter.
[tr. Palmer Bovie (1959)]

If your neighbor's house is burning, your own is next;
for fires, if they're not put out, are apt to spread.
[tr. Fuchs (1977)]

Your own house is in danger when your neighbor's
House is on fire; a fire not watched can spread.
[tr. Ferry (2001)]

It's very much your affair when the house next door is ablaze.
Ignore a fire, and soon you're faced with a conflagration.
[tr. Rudd (2005 ed.)]

If your neighbour’s roof’s in flames, it’s your business too,
And neglected fires have a habit of gaining strength.
[tr. Kline (2015)]