Midrasch zu Bereschit 49:2

Why did he say to them Gather yourselves together, assemble yourselves (Gen. 49:1–2)? He did so to let them realize that they would be exiled twice. After he had designated the elders among them, he began to rebuke them, saying: Reuben, thou art my firstborn (Gen. 49:3). From that very moment Israel merited reciting the Shema. Why? When Jacob was about to depart from this world, he reflected upon it and said: “My grandfather Abraham begot Isaac and Ishmael, and my father, Isaac, begot me and Esau; perhaps there is a blemish in my family.” When they heard this, they proclaimed in unison: Hear, O Israel, the Lord our God, the Lord is one (Deut. 6:4). Hence it is written: Hearken to Israel, thy father (Gen. 49:2).