Midrash for Sanhedrin 5:2
מכות בשלשה משום ר' ישמעאל אומר בעשרים ושלשה עיבור החדש בשלשה עיבור השנה בשלשה דברי ר' מאיר רבן שמעון בן גמליאל אומר בשלשה מתחילין ובחמשה נושאין ונותנין וגומרין בשבעה ואם גמרו בשלשה מעוברת סמיכת הזקנים ועריפת העגלה בשלשה דברי רבי שמעון
Whilst the R. Yosse and R. Aha were together, the former said to the latter: Does it not seem to you that the passage of this half a mile (twilight) lasts but a second? It is certainly my opinion, said R. Aha. However, R. Hiya does not say so, but each twinkling of an eye, measured by the duration of the passage of half a mile (as R. Nehemiah), is doubtful. R. Mena says : I have made an objection in the presence of R. Aha: Have we not learnt elsewhere, that if an impurity is seen, once during the day and again during the intermediate period, or once in the twilight and again on the morrow, when the certainty exists that the impurity dates partly from this day and partly from the next day, there is a certainty as to the circumstances of the impurity, and the sacrifice is obligatory.