In Mahabharata Santi Parva Section CCLXII, is a little long, but worth it:
Whatever fruits one enjoys by penances, by sacrifices, by practising
liberality, by speaking the truth, and by paying court to wisdom, may
all be had by practising the duty of harmlessness. That person who
gives unto all creatures the assurance of harmlessness obtains the
merit of all sacrifices and at last wins fearlessness for himself as
his reward. There is no duty superior to the duty of abstention from
injuring other creatures. He of whom, O great ascetic, no creature is
frightened in the least, obtains for himself fearlessness of all
creatures. He of whom everybody is frightened as one is of a snake
ensconced within one's (sleeping) chamber, never acquires any merit in
this world or in the next. The very gods, in their search after it,
become stupefied in the track of that person who transcends all
states, the person, viz., who constitutes himself the soul of all
creatures and who looketh upon all creatures as identical with his own
self. Of all gifts, the assurance of harmlessness to all creatures is
the highest (in point of merit).
And later on...
Amongst diverse conflicting ordinances, some succeed in comprehending
duty by observing the acts of the good. 3 Why dost thou not consume
them that emasculate bulls and bore their noses and cause them to bear
heavy burthens and bind them and put them under diverse kinds of
restraint, and that eat the flesh of living creatures after slaying
them? Men are seen to own men as slaves, and by beating, by binding,
and by otherwise subjecting them to restraints, cause them to labour
day and night. These people are not ignorant of the pain that results
from beating and fastening in chains.
And keeps going, this is interesting:
In every creature that is endued with the five senses live all the
deities. Surya, Chandramas, the god of wind, Brahman, Prana, Kratu,
and Yama (these dwell in living creatures), There are men that live by
trafficking in living creatures! When they earn a living by such a
sinful course, what scruples need they feel in selling dead carcases?
The goat is Agni. The sheep is Varuna. The horse is Surya. Earth is
the deity Virat. The cow and the calf are Soma. The man who sells
these can never obtain success. But what fault can attach to the sale
of oil, or of Ghrita, or honey, or drugs, O regenerate one?
(BTW here drugs means medicine)
There are many animals that grow up in ease and comfort in places free
from gnats and biting insects. Knowing that they are loved dearly by
their mothers, men persecute them in diverse ways, and lead them into
miry spots abounding with biting insects. Many draft animals are
oppressed with heavy burthens. Others, again, are made to languish in
consequence of treatment not sanctioned by the scriptures.
pay attention here:
I think that such acts of injury done to animals are in no way
distinguished from foeticide.
People regard the profession of agriculture to be sinless. That
profession, however, is certainly fraught with cruelty. The iron-faced
plough wounds the soil and many creatures that live in the soil. Cast
thy eyes, O Jajali, on those bullocks yoked to the plough. Kine are
called in the Srutis the Unslayable. That man perpetrates a great sin
who slays a bull or a cow.
In days of yore, many Rishis with restrained senses addressed Nahusha,
saying, 'Thou hast, O king, slain a cow which is declared in the
scriptures to be like unto one's mother. Thou hast also slain a bull,
which is declared to be like unto the Creator himself. Thou hast
perpetrated an evil act, O Nahusha, and we have been exceedingly
pained at it.' For cleansing Nahusha, however, they divided that sin
into a hundred and one parts and converting the fragments into
diseases cast them among all creatures. Thus, O Jajali, did those
highly-blessed Rishis cast that sin on all living creatures, and
addressing Nahusha who had been guilty of foeticide, said, 'We shall
not be able to pour libations in thy sacrifice.' Thus said those
high-souled Rishis and Yatis conversant with the truths of all things,
having ascertained by their ascetic power that king Nahusha had not
been intentionally guilty of that sin.
These, O Jajali, are some of the wicked and dreadful practices that
are current in this world. Thou practisest them because they are
practised by all men from ancient times, and not because they agree
with the dictates of thy cleansed understanding. One should practise
what one considers to be one's duty, guided by reasons, instead of
blindly following the practices of the world. Listen now, O Jajali, as
to what my behaviour is towards him that injures and him that praises
me. I regard both of them in the same light. I have none whom I like
and none whom I dislike. The wise applauded such a course of conduct
as consistent with duty or religion. Even this course of conduct,
which is consistent with reasons, is followed by Yatis. The righteous
always observe it with eyes possessed of improved vision.'"