I cannot remember where I got this, but I share it as often as I can.
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The Talmud and kindness to animals as a Biblical commandment:
According to the Torah, if we see an animal suffering we are required to help it and, all the more so, it is forbidden to actually cause suffering to animals. While we are permitted to use animals for labor, we are not allowed to overwork an animal. A person is not permitted to eat until he has fed his animals. Even when slaughtering an animal for food, Torah law requires us to avoid causing unnecessary pain to the animal.
There are several different Scriptural sources which deal with different aspects of this teaching. The Talmud (Bava Metzia 32a-b) derives the basic concept from Exodus 23:5, "If you see the donkey of someone you hate lying under its load, you might want to refrain from helping him, but you must make every effort to help him." This is not simply an obligation to help your fellow man, but also an obligation towards the animal itself.
The law that we must feed our animals before we eat is understood from the order of the verse, "I will provide grass in your field for your cattle and you will eat and be satisfied" (Deuteronomy 11:15) which places the feeding of the cattle before our own eating.
The underlying principle of all these laws, as well as the laws of mercy to our fellow human beings, is the commandment "and you walk in His ways" (Deuteronomy 28:9) - to follow the ways of God. In Tehillim (Psalms) 145:9 we are taught, "God is good to all, and His mercy is on all His creations." Just as God is merciful to all His creations, so too must we be merciful to all His creations. In this merit God will have mercy on us. Kindness and mercy are among the defining traits of the Jewish people and we are taught that anyone who shows kindness to animals demonstrates that he is descended from Abraham.
The Talmud tells us that the great sage, Rabbi Yehuda HaNasi, the compiler of the Mishna (the basic framework of the Talmud), was punished for an incident where he was insufficiently caring for an animal, and he was only relieved from his punishment when he showed mercy on some kittens.
There is a story told in the book Charedim by Rabbi Elazar Ezkari (d. 1600) about the Arizal - Rabbi Yitzchak Ashkenazi--the famous mystic of Tzefas, Israel--who was a contemporary of Rabbi Ezkari (who also lived in Tzefas).
The Arizal once looked at the face of a certain Torah scholar and told him, "Your face is marked by the sin of causing pain to animals." The scholar was very disturbed. He investigated the matter and discovered that his wife did not feed their chickens in the morning but instead allowed them to wander through the yard and street to peck for food. He instructed his wife to prepare for them a mixture of bran-flour and water every morning. Shortly after he had done this he met the Arizal again and the Arizal informed him, without knowing about what he had done, that the sin was gone.