Unfortunately I can't find the original source (yet), but according to this commentator, the Jerusalem Talmud states that a portion of a day was as a whole.
The Jerusalem Talmud quotes rabbi Eleazar ben Azariah, who lived around A.D. 100, as saying: “A day and night are an Onah [‘a portion of time’] and the portion of an Onah is as the whole of it” (from Jerusalem Talmud: Shabbath ix. 3, as quoted in Hoehner, 1974, pp. 248-249, bracketed comment in orig.).
The Jerusalem Talmud is also known as the Palestinian Talmud.
PS TheAs the answer to the OP question, the reference (the argument amongst the early sages like Eleazar ben AzariahEleazar ben Azariah) is to the command "be ready on the 3rd day" (Ex. 19:15) with day defined earlier in Gen. 1:5, 8 (dark/evening and light/morning). So, some sages thought in terms of periods (dark or light) to make a day. When is the third day? Some thought at least 4 periods, some 5, some 6 (IshmaelIshmael). One said always 5 (AqivaAqiva). Following are examples.
4 PERIODS
before sunset (1 period and day 1)
night, morning (2 periods (or 3 total) and day 2)
night (1 period (or 4 total) and day 3)
5 PERIODS
night, morning (2 periods and day 1)
night, morning (2 periods (4 total periods) and day 2)
after sunset (1 period (5 total) and day 3)
6 PERIODS
night, morning (2 periods and day 1)
night, morning (2 periods (4 total) and day 2)
night, morning (2 periods (6 total) and day 3)
So, traditional Christianity counts the sign of Jonah as 4 periods to mean 3 days (Fri-Sun).
Some count the full 6 periods (3 days and 3 nights) in ways like Thursday after sunset to Sabbath at sunset.
What is totally wrong is the newish theory of death on Wednesday to either resurrection at Saturday sunset (7 periods or 4 days) or Wednesday to Sunday resurrection (9 periods or 5 days).