The first socialist constitution in human history created by and for the working class was adopted for the newly formed Russian Soviet Federated Socialist Republic (RSFSR) by the Fifth All-Russian Congress of Soviets, July 10, 1918.
From the Great Soviet Encyclopedia 1979:
The RSFSR Constitution of 1918 was the first Soviet constitution. Its fundamental principles were elaborated during the struggle of the working people for Soviet power during the Great October Socialist Revolution of 1917. Many of these principles were reflected in the first decrees of Soviet power and were expressed in documents of the All-Russian Congresses of Soviets, which formulated the most important theses about the Soviet social and state structure, the system of Soviet state bodies, and the legal status of citizens. The Declaration of the Rights of the Working and Exploited People was included as the first section of the RSFSR Constitution of 1918. Like other Soviet constitutions adopted between 1918 and 1935, the RSFSR Constitution of 1918 proclaimed the state of the dictatorship of the proletariat to be the basic instrument of socialist construction and the soviets of workers’, peasants’, and Red Army deputies to be the state form of this dictatorship. State power belonged only to the working people, who made up an absolute majority of the population. The nonworking classes and groups were deprived of their political rights. In comparison with other classes, including the peasantry, the workers were granted certain advantages in electoral rights. For example, at the All-Russian Congress of Soviets the municipal soviets were allotted one delegate per 25,000 electors, whereas the provincial congresses of Soviets were assigned one delegate per 125,000 inhabitants. The RSFSR Constitution of 1918 provided for indirect elections to bodies of state power at the district level and above. The production-territorial principle of elections to the Soviets was established. All of these principles were fundamental to Union republic and autonomous republic constitutions adopted between 1919 and 1929, as well as to the USSR Constitution of 1924, the first Union constitution, which legally affirmed the formation of the USSR in 1922.
The very first line of the constitution's first article read: "Russia is declared to be a republic of the Soviets of Workers', Soldiers', and Peasants' Deputies. All the central and local power belongs to these soviets."
It went on to state:
Bearing in mind as its fundamental problem the abolition of the exploitation of men by men, the entire abolition of the division of the people into classes, the suppression of exploiters, the establishment of a socialist society, and the victory of socialism in all lands, the Third All-Russian Congress of Soviets of Workers', Soldiers', and Peasants' Deputies further resolves:
(a) For the purpose of attaining the socialization of land, all private property in land is abolished, and the entire land is declared to be national property and is to be apportioned among agriculturists without compensation of the former owners, to the measure of each one's ability to till it.
(b) All forests, treasures of the earth, and waters of general public utility, all equipment whether animate or inanimate, model farms and agricultural enterprises, are declared to be national property.
(c) As a first step toward complete transfer of ownership to the Soviet Republic of all factories, mills, mines, railways, and other means of production and transportation, the soviet law for the control of workmen and the establishment of a Supreme Soviet of National Economy is hereby confirmed so as to insure the power of the workers over the exploiters.
(d) With reference to international banking and finance, the Third Congress of Soviets is discussing the soviet decree regarding the annulment of loans made by the Government of the Czar, by landowners and the bourgeoisie, and it trust that the Soviet Government will firmly follow this course until the final victory of the international workers' revolt against the oppression of capital.
(e) The transfer of all banks to the ownership of the Workers' and Peasants' Government, as one of the conditions of the liberation of the toiling masses from the yoke of capital, is confirmed.
(f) Universal obligation to work is introduced for the purpose of eliminating the parasitic strata of society and organizing the economic life of the country.
(g) For the purpose of securing the working class in the possession of complete power, and in order to eliminate all possibility of restoring the power of the exploiters, it is decreed that all workers be armed, and that s Socialist Red Army be organized and the propertied class disarmed.
Full constitution: Constitution of the R.S.F.S.R. (1918) (marxists.org)