杀鸡焉用牛刀
shā​jī​yān​yòng​niú​dāo

why use an ox cleaver to kill a chicken

  1. 杀 殺 shā verb
    to kill
    to kill, slay, or murder; to slaughter animals
  2. 鸡 雞 jī noun
    chicken
    chicken; fowl; bird of the Galliformes order typically raised for its meat or eggs
  3. 焉 yān
    how
    (literary) how; why; by what means; often used in rhetorical questions
  4. 用 yòng verb
    to use
    To use; to employ; to apply or make use of something to achieve a purpose.
  5. 牛刀 niú​dāo
    butcher's knife
    a knife used for slaughtering cattle