“But Jesus answered them.

‘An evil

And adulterous generation

Asks for a sign.

But no sign shall be given

To it,

Except the sign

Of the prophet Jonah.

Just as Jonah was three days

And three nights

In the belly of the sea monster,

Thus,

For three days

And three nights

The Son of man

Will be

In the heart of the earth.’”

ὁ δὲ ἀποκριθεὶς εἶπεν αὐτοῖς Γενεὰ πονηρὰ καὶ μοιχαλὶς σημεῖον ἐπιζητεῖ, καὶ σημεῖον οὐ δοθήσεται αὐτῇ εἰ μὴ τὸ σημεῖον Ἰωνᾶ τοῦ προφήτου.

ὥσπερ γὰρ ἦν Ἰωνᾶς ἐν τῇ κοιλίᾳ τοῦ κήτους τρεῖς ἡμέρας καὶ τρεῖς νύκτας, οὕτως ἔσται ὁ Υἱὸς τοῦ ἀνθρώπου ἐν τῇ καρδίᾳ τῆς γῆς τρεῖς ἡμέρας καὶ τρεῖς νύκτας.

This saying about the sign of Jonah can also be found in Luke, chapter 11:29-30, so that perhaps this is a Q source.  Once again, Q and Matthew went back to their Old Testament roots, using the story of the prophet Jonah, chapter 1:17, where he was in the belly of the whale or the sea monster for 3 days and 3 nights.  The obvious comparison between the 3 days and 3 nights in the belly of the fish and Jesus in the tomb was not lost on the early Christians.  Jesus answered the Pharisees (ὁ δὲ ἀποκριθεὶς εἶπεν αὐτοῖς).  He called them an evil and adulterous generation (Γενεὰ πονηρὰ καὶ μοιχαλὶς).  Why were they asking for or seeking signs (σημεῖον ἐπιζητεῖ)?  The only sign they were going to get was the sign of the prophet Jonah (καὶ σημεῖον οὐ δοθήσεται αὐτῇ εἰ μὴ τὸ σημεῖον Ἰωνᾶ τοῦ προφήτου).  He had been in the belly of the sea monster or whale that swallowed him for 3 days and 3 nights (ὥσπερ γὰρ ἦν Ἰωνᾶς ἐν τῇ κοιλίᾳ τοῦ κήτους τρεῖς ἡμέρας καὶ τρεῖς νύκτας).  Thus, the Son of Man would be 3 days and 3 nights in the heart of the earth (οὕτως ἔσται ὁ Υἱὸς τοῦ ἀνθρώπου ἐν τῇ καρδίᾳ τῆς γῆς τρεῖς ἡμέρας καὶ τρεῖς νύκτας).  This was a clear allusion to the death of Jesus in the tomb for 3 days and 3 nights before his resurrection.