What is the genetic basis for proof that codon is a triplet?

Explanation

George Gamow, a physicist argued that since there are only 4 bases and if they have to code for all the 20 amino acids, the code should be made of three nucleotides. This was a very bold proposition because a permutation combination of 4(4 x 4 x 4) would generate 64 codons. This proposition was later proved by Khorana and Nirenberg If the codon consists of 4 letters (4 x 4). only sixteen codes are possible which is less than the number of amino acids. As the ribosomes move on mRNA the codons are read in a contiguous fashion