Ultimately, the bond of all companionship, whether in marriage or friendship, is conversation.

Oscar Wilde (1854-1900) Irish poet, wit, dramatist

Letter (1897-03) to Alfred Douglas, “Epistola: In Carcere et Vinculis”
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Wilde titled the letter, written while in prison in Reading, England, Epistola: In Carcere et Vinculis ("Letter: In Prison and in Chains"). Upon his release, the letter was entrusted to Robert Ross, who in 1905, after Wilde's death, published an edited version under the title De Profundis ("From the Depths," from Psalm 130), and later editions have retained that name.

This quotation was not in the 1905 edition, but in the eventually fully-restored version in Wilde's complete letters.

More information on the history of the letter here.