“Think of your forefathers and posterity.”

He here is translating a phrase of Calgacus in Vita Agricolae by Tacitus : Et majores et posteros cogitate.
Oration at Plymouth (1802)
Context: The barbarian chieftain, who defended his country against the Roman invasion, driven to the remotest extremity of Britain, and stimulating his followers to battle, by all that has power of persuasion upon the human heart, concludes his exhortation by an appeal to these irresistible feelings — "Think of your forefathers and of your posterity."

Oration at Plymouth (1802)
Context: Among the sentiments of most powerful operation upon the human heart, and most highly honorable to the human character, are those of veneration for our forefathers, and of love for our posterity. They form the connecting links between the selfish and the social passions. By the fundamental principle of Christianity, the happiness of the individual is Later-woven, by innumerable and imperceptible ties, with that of his contemporaries: by the power of filial reverence and parental affection, individual existence is extended beyond the limits of individual life, and the happiness of every age is chained in mutual dependence upon that of every other.

Vol. I : The Dedication (March 1772)
Institutes of Natural and Revealed Religion (1772–1774)
Context: Respect a parliamentary king, and chearfully pay all parliamentary taxes; but have nothing to do with a parliamentary religion, or a parliamentary God.
Religious rights, and religious liberty, are things of inestimable value. For these have many of our ancestors suffered and died; and shall we, in the sunshine of prosperity, desert that glorious cause, from which no storms of adversity or persecution could make them swerve? Let us consider if as a duty of the first rank with respect to moral obligation, to transmit to our posterity, and provide, as far as we can, for transmitting, unimpaired, to the latest generations, that generous zeal for religion and liberty, which makes the memory of our forefathers so truly illustrious.

Speech in the House of Commons (3 June 1862)
1860s

Ce qu'on appelle la postérité, c'est la postérité de l'œuvre.
Source: In Search of Lost Time, Remembrance of Things Past (1913-1927), Vol II: Within a Budding Grove (1919), Ch. I: "Madame Swann at Home"

Napoleon : In His Own Words (1916)

Speech, Senate (29 January 1850).

No. 587 (20 August 1714).
The Spectator (1711–1714)

Quoted by Z. Chafee Jr. Atlantic Monthly (January 1955)