Midrash Tanchuma, Nasso 26.
Sefaria — Midrash Tanchuma, Nasso 26
מָה בֵּרְכוֹ? שֶׁיַּעֲלֶה נִילוּס לְרַגְלָיו.
English: What blessing did he give him? That the Nile should rise at his feet.
On the daf / in context: The existing Sifrei Devarim 38 attribution was too broad and does not establish this specific Nile-blessing tradition. Midrash Tanchuma, Nasso 26 gives the specific source.
What Is Learned: Yaakov’s blessing is interpreted as a blessing that the Nile would rise for Egypt, bringing the water needed for the land.