Source, in full
response by Tolkien during a "Hobbit Meal" at Rotterdam Holland attended 28 March 1958, in response to Professor Lambers summarizing approximately (as translated by Rossenberg from Dutch) "Gollum is mollified by the vulnerability of the sleeping hobbit and is at the point of redemption. But Sam, misguided by the love for his master, intervenes and thus prevents the rebirth of Smeagol. Sam’s goodness makes the goodness of Gollum impossible."
Notes from Wikiquote
René van Rossenberg, "Tolkien's Exceptional Visit to Holland: A Reconstruction", Mythlore: A Journal of J.R.R. Tolkien, C.S. Lewis, Charles Williams, and Mythopoeic Literature, Vol. 21: No. 2 (1996), Article 45 | Speaking about the same scene he wrote about in his January 1945 letter to Christopher, and would later call 'most tragic' in September 1963