Genesis 38:10 (KJV)

And she yet again conceived, and bare a son; and called his name Shelah: and he was at Chezib, when she bare him.

And Judah took a wife for Er his firstborn, whose name was Tamar.

And Er, Judah's firstborn, was wicked in the sight of the LORD; and the LORD slew him.

And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother.

And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother.

Then said Judah to Tamar his daughter in law, Remain a widow at thy father's house, till Shelah my son be grown: for he said, Lest peradventure he die also, as his brethren did. And Tamar went and dwelt in her father's house.

And in process of time the daughter of Shuah Judah's wife died; and Judah was comforted, and went up unto his sheepshearers to Timnath, he and his friend Hirah the Adullamite.

And it was told Tamar, saying, Behold thy father in law goeth up to Timnath to shear his sheep.

And she put her widow's garments off from her, and covered her with a vail, and wrapped herself, and sat in an open place, which is by the way to Timnath; for she saw that Shelah was grown, and she was not given unto him to wife.

When Judah saw her, he thought her to be an harlot; because she had covered her face.