He who plans death for his foe should know well that a like death is planned for himself to share in.

[Qui alteri exitium parat,
Eum scire oportet sibi paratam pestem ut participet parem.]

Ennius (239–169 BC) Roman poet, writer [Quintus Ennius]

Hectoris Lytra [The Ransom of Hector], fragment [tr. Warmington (Loeb) (1913)]
    (Source)

This fragment is known primarily through Cicero, in his Tusculan Disputations [Tusculanae Disputationes], Book 2, ch. 17 (2.17) / sec. 39 (2.39) (45 BC). Cicero references these lines as being from a play, usually identified as being by Ennius, either his Achilles (or Achilles Arestarchi) or, more commonly, his Hectoris Lytra.

Ennius' inspiration for the larger passage quoted by Cicero (a conversation between Eurypylus and Patroclus) comes from Homer, Illiad, Book 11. Eurypylus here explains why a warrior should bear his injuries with stoic patience.

(Source (Latin)). Other translations:

Who another doth a mortal blow intend,
Must know, like hand lift up, him to offend.
[tr. Wase (1643)]

Who at his enemy a stroke directs,
His sword to light upon himself expects.
[tr. Main (1824); Yonge (1853)]

Whoever aims destruction at another,
Should know, that plagues are brewing for himself,
That he may share the like.
[tr. Otis (1839)]

The man who wields the implements of death,
Should marvel not if they are turned against him.
[tr. Peabody (1886)]

He who is preparing destruction for another may be certain that his own life is in danger.
[tr. Taylor/Hunt (1916)]

He who prepares death for another should know that it is in store for him that he shall partake of a like destruction.
[tr. Douglas (1990)]

The man who contrives another's death should know a like is prepared for him, that he may share an equal fate.
[tr. Davie (2017)]