Which is more correct: "I love you by Allah" or "I love you in Allah," and what is the difference between them?

The correct phrasing for a Muslim to say to his brother is: "I love you for the sake of Allah (أحبك في الله)". This is established from the Companions, the Tabi'un (followers of the Companions), and those who followed them. The Prophet (peace be upon him) commanded that one should inform those whom one loves of this, and it has been reported from the early generations (Salaf) that they used to say it. As for the phrasing: "I love you by Allah (أحبك بالله)," it has not been reported from the Salaf, and the letter "ba" (ب) has multiple meanings.