Hyginus Fabulae, 77-78
Leda / Tyndareus.

leda. Iuppiter Ledam Thestii filiam in cygnum conuersus ad flumen Eurotam compressit, et ex eo peperit Pollucem et Helenam, ex Tyndareo autem Castorem et Clytaemnestram.

tyndarevs. Tyndareus Oebali filius ex Leda Thestii filia procreauit Clytaemnestram et Helenam; Clytaemnestram Agamemnoni Atrei filio dedit in coniugium; Helenam propter formae dignitatem complures ex ciuitatibus in coniugium proci petebant.
Tyndareus cum ab Agamemnone repudiaret filiam suam Clytaemnestram uereretur timeretque ne quid ex ea re discordiae nasceretur, monitus ab Vlixe iureiurando se obligauit et arbitrio Helenae posuit ut cui uellet nubere coronam imponeret. Menelao imposuit, [ut] cui Tyndareus eam dedit uxorem regnumque moriens Menelao reliquit.


Jupiter turned himself into a swan and lay with Leda, the daughter of Thestius, on the bank of the Eurotas, and from that union he begat Pollux and Helen, and from Tyndareus were born Castor and Clytemnestra.

Tyndareus, the son of Oebalus, begot from Leda, the daughter of Thesius, Clytemnestra and Helen. He gave Clytemnestra to Agamemnon, the son of Atreus, in marriage; large numbers of suitors from foreign lands sought to marry Helen on account of her reputation of her beauty. When Tyndareus rejected his own daughter Clytemnestra back from Agamemnon, and feared deeply that some disagreement would arise from this matter, Tyndareus followed Ulysses' recommendation and bound himself with an oath. He decreed a judgment upon Helen that he would place the crown upon anybody who wished to marry her. And upon Menelaus Tyndareus determined that he gave her to him to wife and at his own death left the kingdom for Menelaus.
—— trans by RTM