Answer: "Slowly, silently, now the moon"
Step-by-step explanation: While there are some other instances of alliteration in the poem, in the first line is the finest example, because the two very first words begin with the "s" sound, just like the title, "Silver." By emphasizing these two words with this alliteration, the author conveys a "nocturnal" mood that will continue to develop for the rest of the poem.