Midrash su Esodo 4:26

"When you shall go out... When you shall happen upon a birds nest before yo..." (Deuteronomy 22:6) - Law: What is the law if an infant was born circumcised, may one circumcise him? So taught the sages: An infant born circumcised requires a prick of covenantal blood from him because of the covenant of Abraham. Where do you learn this from? From the Torah, as it says, (Genesis 17:13) "Whether born in your household or bought with your money... [they must be circumcised]." Another explanation:

Then sang Moses. At times the word then refers to events that happened in the past, and at other times it refers to events that are to transpire in the future. The word then refers to past events in the following verses: Then began man to call upon the name of the Lord (Gen. 4:26), Then she said: “A bridegroom of blood” (Exod. 4:26), Then sang Israel this song (Num. 21:17), Then Solomon spoke (I Kings 18:12). All of these refer to past events. Instances in which the word then alludes to future events are: Then thou shalt see (Isa. 60:5), Then shall thy light break forth (ibid. 58:8), Then shall the lame man leap as a hart (ibid. 35:6), Then the eyes of the blind shall be opened (ibid., v. 5), Then shall the virgin rejoice (Jer. 31:13), Then was our mouth filled with laughter, and our tongue with singing (Ps. 126:2), and Then said they among the nations (ibid.). All these allude to future events. From this we learn that the concept of the resurrection of the dead is derived from the Torah.