Sforno on Exodus 4:25/ספורנו על שמות ד׳:כ״ה

1ותאמר כי חתן דמים אתה לי. I have done this seeing that when I became married to you and you were my bridegroom you stipulated that our sons would have to be circumcised and that we would extract a certain amount of blood as the mark of the covenant with G’d. Tzipporah said all this to the angel who wanted to kill her husband in his defence i.e. the angel must know that there was no deliberate negligence in performing the rite of circumcision.