Rambam - Sefer HaMitzvos: Negative Mitzvah 113: Negative Mitzvah 113

Negative Mitzvah 113: We are forbidden to use any animal for work that has been designated for sacrifice
Deuteronomy 15:19 "You shall not do any work with the first-born of your ox"

Sammy and Yudi were busy constructing an indoor tent in their playroom. Upon completing the frame, they began searching for a large covering to place over the frame and complete the tent.

Yudi's eyes fell upon his father's tallit bag.

"Hey! Sammy," he cried, "I have a perfect idea! Daddy's `tallit' is large and we can use the `tzitzit' to tie it to the chair legs."

Sammy looked at his younger brother in disbelief.

"You can't do that!", he said in shock. "A tallit is a not just another piece of cloth! It's considered holy because it's used for prayer to HaShem!"

Yudi immediately realized his mistake and they found something else to use for a tent.

At the time of the Exodus from Egypt, HaShem claimed that all first-born males and animals are sacred to Him.

There are many Mitzvot that apply to the first - born (see Positive Mitzvot 79-82).

Just as the Torah forbids us to use first-born animals for our own purposes, so, too, we cannot do work with any other animals set aside for sacrifice. For example, we may not harness them to plow our fields or burden them to carry our loads.

Abraham was a tzaddik of a different mold altogether. G-d told Abraham He was about to destroy the cities of Sodom and Gemora -- cities corrupt and evil to the core. Abraham argued, "Perhaps there are righteous people there! Will the Judge of All the Earth not do justice?" Abraham felt a sense of ownership for the world in which he lived. If there was something wrong, it needed to be changed. Even if it had been decreed by the will of G-d.

From: Bringing Heaven Down to Earth by Tzvi Freeman - tzvif@aol.com