“Socialism without democracy is pseudo-socialism, just as democracy without socialism is pseudo-democracy.”

No Compromise – No Political Trading (1899)

Speech to the European Republic Committee at the American Club in London (25 October 1978), quoted in The Times (26 October 1978), p. 5
1970s

Attributed to Marx in recent years, including in Communism (2007) by Tom Lansford, p. 48, but the earliest occurrence of this yet located is in The Communist Review (1952) by the Communist Party of Great Britain, p. 15, where it is used to characterize the Communist agenda.
Disputed

Voprosi Leninizma, Gosudarstvennoe izdatelstvo politicheskoy literaturi, (1939)
Stalin's speeches, writings and authorised interviews

Source: 2010s, Why Marx Was Right (2011), Chapter 9, p. 202

12 September 1848, "Discours prononcé à l'assemblée constituante le 12 Septembre 1848 sur la question du droit au travail", Oeuvres complètes, vol. IX, p. 546 https://fr.wikisource.org/wiki/Page:Tocqueville_-_%C5%92uvres_compl%C3%A8tes,_%C3%A9dition_1866,_volume_9.djvu/564; Translation (from Hayek, The Road to Serfdom):
Original text:
La démocratie étend la sphère de l'indépendance individuelle, le socialisme la resserre. La démocratie donne toute sa valeur possible à chaque homme, le socialisme fait de chaque homme un agent, un instrument, un chiffre. La démocratie et le socialisme ne se tiennent que par un mot, l'égalité; mais remarquez la différence : la démocratie veut l'égalité dans la liberté, et le socialisme veut l'égalité dans la gêne et dans la servitude.
1840s

Source: Two Hundred Million Americans in Search of a Government (1969), p. 46

“Concerning the International Situation,” Works, Vol. 6, January-November, 1924, pp. 293-314.
Stalin's speeches, writings and authorised interviews
Context: Social democracy is objectively the moderate wing of fascism.... These organisations (ie Fascism and social democracy) are not antipodes, they are twins.

As quoted in World Marxist Review : Problems of Peace and Socialism (1958), p. 40
Context: Democracy and socialism are means to an end, not the end itself. We talk of the good of society. Is this something apart from, and transcending, the good of the individuals composing it? If the individual is ignored and sacrificed for what is considered the good of the society, is that the right objective to have?
It was agreed that the individual should not be sacrificed and indeed that real social progress will come only when opportunity is given to the individual to develop, provided "the individual" is not a selected group but comprises the whole community. The touchstone, therefore, should be how far any political or social theory enables the individual to rise above his petty self and thus think in terms of the good of all. The law of life should not be competition or acquisitiveness but cooperation, the good of each contributing to the good of all.