matthew 12:40 For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth. - Free Bible Online

      40. For as Jonas was--"a sign unto the Ninevites, so shall also the Son of man be to this generation" (Lu 11:30). For as Jonas was
      three days and three nights in the whale's belly-- (Jon 1:17).
      so shall the Son of man be three days and three nights in the heart of the earth--This was the second public announcement of His resurrection three days after His death. (For the first, see Joh 2:19). Jonah's case was analogous to this, as being a signal judgment of God; reversed in three days; and followed by a glorious mission to the Gentiles. The expression "in the heart of the earth," suggested by the expression of Jonah with respect to the sea (2:3, in the Septuagint), means simply the grave, but this considered as the most emphatic expression of real and total entombment. The period during which He was to lie in the grave is here expressed in round numbers, according to the Jewish way of speaking, which was to regard any part of a day, however small, included within a period of days, as a full day. (See 1Sa 30:12, 13; Es 4:16; 5:1; Mt 27:63, 64, &c.).

JFB.