Genesis 38:9

וַיֵּ֣דַע אוֹנָ֔ן כִּ֛י לֹּ֥א ל֖וֹ יִהְיֶ֣ה הַזָּ֑רַע וְהָיָ֞ה אִם־בָּ֨א אֶל־אֵ֤שֶׁת אָחִיו֙ וְשִׁחֵ֣ת אַ֔רְצָה לְבִלְתִּ֥י נְתָן־זֶ֖רַע לְאָחִֽיו׃

γνοὺς δὲ Αύνὰν ἣτι οὐκ αὐτῷ ἔσται τὸ σπέρμα, ἐγίνετο, ὃταν εἰσήρχετο πρὸς τὴν γυναῖκα τοῦ ἀδελφοῦ αὐτοῦ, ἐξέχεεν ἐπὶ τὴν γῆν τοῦ μὴ δοῦναι σοέρνα τῷ ἀδελφῷ αὐτοῦ.

Greek text from Swete's edition, CC BY-SA 4.0

Y sabiendo Onán que la simiente no había de ser suya, sucedía que cuando entraba á la mujer de su hermano vertía en tierra, por no dar simiente á su hermano.

But Onan knew that the offspring would not belong to him; so whenever he would sleep with his brother’s wife, he would spill his seed on the ground so that he would not produce offspring for his brother.