זוהר שמות — דף קסח ע”א

Abel was born from the masculine side but the accuser weakened and the virile portion was included in great suffering. Cain was born from the side of Gevurah on the left, and Abel from the side of Gevurah on the right. There is a deep secret in letters added to or missing from names, and the holy text alludes there to a supreme matter. The letter He is the secret of the delicacy of the supernal neshamah hidden in the supernal world. The secret of the names that are bound in the influence of this hidden He letter reveals the depth of the secret.