who says A, must say B
who
hu:
hoo
says
seɪz
seiz
A,
eɪ
ei
must
mʌst
mast
say
seɪ
sei
B
bi:
bi

英語での「who says A, must say B」の定義と意味

who says a, must say b
01

used to suggest that if someone starts or initiates a particular action or process, it is expected that they will continue and complete the subsequent steps 

ことわざ
例
If you promise to do something, you must follow through with it. As the saying goes, who says A, must say B. 
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