Mitzvah 243: The commandment of love of Israel

The commandment of love of Israel: To love [with] love of the soul each one of Israel - meaning to say that we have compassion for an Israelite and for his money, [just] like a person has compassion for himself and for his [own] money; as it stated (Leviticus 19:18), “you shall love your neighbor as yourself.” And they, may their memory be blessed, said (Shabbat 31a), “What is hateful to you, do not do to your fellow.” And they said in Sifra, Kedoshim, Chapter 4:12, “Rabbi Akiva said, ‘This is a great principle in the Torah’” - meaning to say that many commandments are dependent upon it. As one that loves his fellow like himself will not steal his money, have adultery with his wife, cheat his money from him nor hurt him from any angle. And so [too,] are there several other commandments dependent on this - the thing is well-known [revealed] to all who have intellect.

Sefer HaChinukh cites Leviticus 19:18.

you shall love your neighbor as yourself.

The root of the commandment is well-known - as in the way that he acts to his fellow, so will his fellow act to him. And there will be peace among the creatures with this.

And the laws of this commandment are included in the commandment, as the general principle of everything is that a man behave with his fellow in the way that a man behaves [with] himself - to guard his money and to distance all injury from him. And if he recounts things about him, he recounts them for praise, and he relates to his honor; and he does not become honored through his disgrace - and as they, may their memory be blessed, said (Talmud Yerushalmi Chagigah 2:1), “One who is honored by the disgrace of his fellow has no share in the world to come, but one who treats his fellow with love, peace and neighborliness, seeks their benefit and is happy about their good, the verse states about him, ‘Israel, through you will I be glorified’ (Isaiah 49:3).”

And this commandment is practiced in every place and at all times. And one who transgresses it and is not careful about the money of his fellow, to guard it - and all the more so, if he injures him with money or caused him pain in any matter, volitionally - has violated this positive commandment; besides the liability that there is in it according to the matter in which he injured him, as is explained in its place.

• Shabbat 31a
• Isaiah 49:3