Reference for Pesachim 121:10

ר' אומר לשון סורסי הוא כאדם שאומר לחבירו כוס לי טלה זה

You might think that if he slaughtered it for those who were not registered for it, he should be as one who violates the precept, yet it is fit. Therefore it stated, 'according to the number of [be-miksath] [the souls]. ye shall make your count [takosu]': the Writ reiterated it, to teach that it is indispensable.

Jastrow