Niddah, Chapter 10, Mishnah 2. This mishnah addresses a young bride, and the question of when blood that she sees is attributed to the wound of her besulim rather than to niddah. Beis Shammai and Beis Hillel argue in three separate cases, each one turning on how physically mature the bride was when she married.
A bride whose time to see has not yet arrived
The first case: "Tinokes shelo higia zemanah liros veniseis" - a young girl who has not yet reached the stage of physical maturity at which a woman would be expected to menstruate, and she marries.
Beis Shammai say "nosnin lah arba leilos": we grant her four nights. Any blood she sees on those first four nights of her marriage is attributed to the tearing of the hymen, that is, to a wound, and it is not treated as the blood of niddah.
Beis Hillel say "ad shetichyeh hamakah": until the wound heals. As long as the wound remains open, we continue to assume that the blood is coming from it, and not from niddah.
A bride who has reached maturity
The second case: "higia zemanah liros veniseis" - she has already reached the stage at which a girl's body is developed enough for menstruation to be expected of her, though in practice she has never yet seen blood, and while in that condition she marries.
Beis Shammai say "nosnin lah lailah rishon": she is granted the first night in its entirety. Anything she finds over the course of that night is credited to the wound of the besulim, and she is not rendered a niddah by it.
Beis Hillel disagree: "ad motzaei Shabbos, arba leilos" - she is granted until Motzaei Shabbos, Saturday night, which comes out to four nights. The calculation assumes that she married on Wednesday, which is the day the Mishnah in Kesuvos gives for the marriage of a besulah. Counting Wednesday night through Motzaei Shabbos yields four nights, and throughout them any blood she sees is attributed to her besulim and not to niddah.
A bride who had already begun to see blood
The third case: "Ra'asah ve'odah beveis aviha" - her periods had already started before the marriage, back when she was still a girl under her father's roof.
Beis Shammai say "nosnin lah be'ilas mitzvah": only the very first marital act is covered. Blood that emerges from it is attributed to the wound of the besulim.
Beis Hillel say "kol halailah shelah": the allowance is not limited to that one act; the night as a whole is hers. Blood that appears at any point before morning is still ascribed to the wound of the besulim, and she is not a niddah on its account.
In all three cases, then, the debate is the same one: how long may we go on crediting the wound with the blood we find, before the blood must be treated as the blood of a niddah. Beis Shammai measure by fixed allotments (four nights, one night, a single act), while Beis Hillel measure more generously in each case (until the wound heals, four nights, the entire night).