Tractate Niddah - Chapter 1 - Mishnah 6

And of what did they speak when they said that "it suffices [for them to reckon] their period of uncleanness from the time [of their discovering of the flow]"? [This refers to] the first observation, but after a second observation she defiles retroactively for a period of twenty-four hours. If she saw the first flow on account of an accident even it suffices for her even a subsequent observation [to reckon her uncleanness from] the time of her [observing of the flow].