“Legislative reform and revolution are not different methods of historic development that can be picked out at the pleasure from the counter of history, just as one chooses hot or cold sausages. Legislative reform and revolution are different factors in the development of class society. They condition and complement each other, and are at the same time reciprocally exclusive, as are the north and south poles, the bourgeoisie and proletariat.”

Source: Reform or Revolution (1899), Ch.8

“Lessons of the Commune”, in Zagranichnaya Gazeta, No. 2 (23 March 1908) http://www.marx.org/archive/lenin/works/1908/mar/23.htm, as translated by Bernard Isaacs, Collected Works, Vol. 13, p. 478.
1900s
Variant: The proletariat should not ignore peaceful methods of struggle — they serve its ordinary, day-to-day interests, they are necessary in periods of preparation for revolution — but it must never forget that in certain conditions the class struggle assumes the form of armed conflict and civil war; there are times when the interests of the proletariat call for ruthless extermination of its enemies in open armed clashes. This was first demonstrated by the French proletariat in the Commune and brilliantly confirmed by the Russian proletariat in the December uprising.

Source: The Limits of State Action (1792), Ch. 16

Source: Object-oriented design: With Applications, (1991), p. 56: Booch is citing: Cox, B. 1986. Object-Oriented Programming An Evolutionary Approach. Reading, MA: Addison-Wesley, p. 69.

Source: Reform or Revolution (1899), Ch. 7

For the Centenary of Lenin’s Birth (1971) (excerpts)

As quoted in The Myth of the Nation and the Vision of Revolution: The Origins of Ideological Polarization in the 20th Century, Jacob Talmon, University of California Press (1981) p. 487
Undated

Principles of Communism (1847)
Context: Everywhere the proletariat develops in step with the bourgeoisie. In proportion, as the bourgeoisie grows in wealth, the proletariat grows in numbers. For, since the proletarians can be employed only by capital, and since capital extends only through employing labor, it follows that the growth of the proletariat proceeds at precisely the same pace as the growth of capital. Simultaneously, this process draws members of the bourgeoisie and proletarians together into the great cities where industry can be carried on most profitably, and by thus throwing great masses in one spot it gives to the proletarians a consciousness of their own strength. Moreover, the further this process advances, the more new labor-saving machines are invented, the greater is the pressure exercised by big industry on wages, which, as we have seen, sink to their minimum and therewith render the condition of the proletariat increasingly unbearable. The growing dissatisfaction of the proletariat thus joins with its rising power to prepare a proletarian social revolution.

Diary (14 February 1879)
Diary and Letters of Rutherford Birchard Hayes (1922 - 1926)