מדרש אויף בראשית 38:16

(Gen. 38:15:) WHEN JUDAH SAW HER, HE THOUGHT HER TO BE A HARLOT BECAUSE SHE HAD COVERED HER FACE. However, R. Johanan said: SHE COVERED HER FACE as long as she was in her father-in-law's house.⁵⁵Therefore Judah did not recognize her. From here our masters have said: A man must be acquainted with his daughter-in-law.⁵⁶Gen. R. 85:8. Judah said: This is a harlot. What concern do I have about her? He went on. When he had gone on, she raised her eyes to the Holy One. She said to him: Sovereign of the World, am I to go away empty from the body of this righteous man? Immediately the Holy One sent Michael and brought him back. Where is it shown? Where it is written here (in vs. 16): SO HE TURNED ASIDE UNTO HER. And it is written (in Numb. 22:23): THEN THE SHE-ASS SAW THE ANGEL OF THE LORD…. SO THE SHE-ASS TURNED ASIDE.⁵⁷Since the verb “turned aside” appears where an angel causes the turning aside in Numb. 22:23, an angel must be involved where the same verb appears in Gen. 38:16. (Gen. 38:16, cont.:) AND HE SAID UNTO HER: HEY THERE, PLEASE < LET ME COME UNTO YOU >. Why? (Ibid., cont.:) BECAUSE HE DID NOT KNOW THAT SHE WAS HIS DAUGHTER-IN-LAW. (Vs. 17:) AND HE SAID: I WILL SEND YOU A GOAT KID. He said to her (in vs. 18): WHAT PLEDGE SHALL I GIVE YOU? AND SHE SAID: YOUR SEAL…. THEN HE CAME UNTO HER, AND SHE CONCEIVED < FOR HIM >. Scripture lacks nothing; so what is the meaning of SHE CONCEIVED FOR HIM? It simply means that she gave birth to kings like him. (Vs. 24:) NOW IT CAME TO PASS ABOUT THREE MONTHS LATER…. R. Judah bar Shallum the Levite said: After three months it is discernible whether a woman is pregnant or not.⁵⁸Yev. 4:1 (10a); TNid. 1:7; Nid. 8b; yNid. 1:4 (49a); Gen. R. 85:10.

“Judah saw her and thought her to be a harlot, because she covered her face” (Genesis 38:15).
“Judah saw her” – Rabbi Ḥiyya bar Zavda said: A person must be very careful regarding his wife’s sister and his female relatives, that he not stumble with one of them. From whom do you derive it? From Judah – “Judah saw her…” Why? It is “because she covered her face” while she was still in her father-in-law’s house.⁴¹Yehuda did not recognize Tamar because she had always kept her face covered in his house. This was a sign of modesty, which should be emulated (Yefe To’ar). Alternatively, the midrash is saying one should take care to recognize his female relatives. Yehuda did not recognize Tamar because she had always kept her face covered in his house, and that is why he mistook her for a harlot (Etz Yosef).
“He turned to her by the road, and he said: Please, let me consort with you, for he did not know that she was his daughter-in-law. She said: What will you give me that you would consort with me” (Genesis 38:16)?
Another matter: “Judah saw her” – he did not pay attention to her. When she covered her face, he said: Were she a harlot, would she have covered her face?⁴²Originally, when Judah saw her from a distance, he took her to be a harlot, and therefore was going to distance himself from her. But then she covered her face, leading Judah to believe she was not a harlot, and he did not distance himself from her. Rabbi Yoḥanan said: He sought to pass, but the Holy One blessed be He dispatched to him the angel that is responsible for desire. He said to him: ‘Judah, where are you going? From where will kings be produced, from where will the prominent ones be produced?’ “He turned to her by the road” – despite himself, against his will.