Bereshit Raba 33:3

It happened in the days of Rabbi Tanchuma that the people of Israel needed rain. They approached Rabbi Tanchuma saying, “May the Rabbi decree a Ta'anit (Fast Day)?” He decreed a Ta'anit. The first day went by, so did the second and the third but rain didn't come. Rabbi Tanchuma went into the Beit Midrash and told the people, “My Sons, you should fill yourself up with mercy for one another and Hashem will have mercy on you.” They started to give Tzedaka to the poor. As they were doing so, they noticed a man giving money to a woman he had divorced. They approached Rabbi Tanchuma and told him: Why should we do if we see other people sin? The Rabbi asked them, “What did you see?” They told him, “We have seen a man giving money to a woman he had divorced.” The Rabbi sent for the person. As the person arrived he asked him, “What is your relationship with the woman?” The man answered, “She is my divorced wife.” The Rabbi asked, “Why did you give her money?” The man answered, “Rabbi, I've seen her in trouble and had mercy for her.” At this point Rabbi Tanchuma raised his hands towards heaven and said, “Master of the Universe, See this man who had no financial obligation to that woman, but as he noticed she's in trouble, had mercy for her; you are known to have Mercy, and we are the children of Abraham, Yitzchak and Yaakov. Shouldn't you have mercy for us?” Immediately rain started to fall and the land was watered.