We are obligated to be more scrupulous in fulfilling the commandment of charity than any other positive commandment because charity is the sign of a righteous man, the seed of Abraham our Father.

Maimonides (1135-1204) Spanish Jewish philosopher, scholar, astronomer, physician [Moses ben Maimon, Rambam, רמב״ם]

Mishneh Torah [Repetition of the Torah] / Sefer Yad ha-Hazaka [The Strong Hand], Book 7 “Seeds [Sefer Zeraim],” ch. 10, #1 (1180)
    (Source)

As quoted in Isadore Twersky, ed., A Maimonides Reader (1972), and the most common English translation. Alternate translation:

We must be especially careful to observe the mitzvah of tzedakah, more so than any other positive mitzvah, for tzedakah is a sign of the righteous [tzadik] lineage of Abraham, our father.
[tr. Meszler (2003)]

More discussion of the connotations of the Hebrew tzedakah that are not covered by the English charity here: Tzedakah - charity in the Jewish tradition - BJE