Mishnà su Amos 9:78

Rabbi Halafta of Kefar Hanania said: when ten sit together and occupy themselves with Torah, the Shechinah abides among them, as it is said: “God stands in the congregation of God” (Psalm 82:1). How do we know that the same is true even of five? As it is said: “This band of His He has established on earth” (Amos 9:6). How do we know that the same is true even of three? As it is said: “In the midst of the judges He judges” (Psalm 82:1) How do we know that the same is true even of two? As it is said: “Then they that fear the Lord spoke one with another, and the Lord hearkened, and heard” (Malachi 3:16). How do we know that the same is true even of one? As it is said: “In every place where I cause my name to be mentioned I will come unto you and bless you” (Exodus 20:21).

On that day Judah, an Ammonite convert, came and stood before them in the house of study. He said to them: Do I have the right to enter into the assembly? Rabban Gamaliel said to him: you are forbidden. Rabbi Joshua said to him: you are permitted. Rabban Gamaliel said to him: the verse says, "An Ammonite or a Moabite shall not enter into the assembly of the Lord: even to the tenth generation" (Deuteronomy 23:4). R. Joshua said to him: But are the Ammonites and Moabites still in their own territory? Sanheriv, the king of Assyria, has long since come up and mingled all the nations, as it is said: "In that I have removed the bounds of the peoples, and have robbed their treasures, and have brought down as one mighty the inhabitants" (Isaiah 10:1. Rabban Gamaliel said to him: the verse says, "But afterward I will bring back the captivity of the children of Ammon," (Jeremiah 49:6) they have already returned. Rabbi Joshua said to him: [another] verse says, "I will return the captivity of my people Israel and Judah" (Jeremiah 30:3). Yet they have not yet returned. So they permitted him to enter the assembly.