This is a part of upanisad which explains the importance of cows.
Saudäsa uväca

trailakya bhagavan kinsvit
pavitram kathyate’ nagha
yat kirtayan sadä martyaù
präpnuyät punyam uttamam
Saudäsa said, “O my lord! O sinless sage, what is that pure object within the three worlds that simply by taking its name, human beings can accumulate great piety?”

Vasistha uvaca

gävaù surabhi gandhinyas
tathä guggulu gandhayaù
gävaù pratiñöhä bhütänäà
gävaù svastyanam mahat
The great sage Vasistha said, “O King, many varieties of aroma emanate from the body of a cow. As such, many cows are as fragrant as guggula resin. The cow is the support of all living entities, and the ocean of auspiciousness.”

gävo bhütäm ca bhävyaà ca
gävah pustih sanätané
gävo laksmäs tathä mülaà
goñu dattam na naçyati
“The cow is the past and future. She nourishes the health of all living entities, and she is the root of prosperity. The piety one achieves by feeding a cow in never destroyed.”

annaà hy paramam gävo
devänäà paramaà haviù
svähakära vañaö kärau
goñu nityaà pratiñöhitau
“The cow is the cause of one’s accumulation of food grains. She awards the best sacrificial ingredients to the demigods. The sacrifice of the demigods and the sacrifice of Indra are both performed on the basis of the cow.”

gävo yajïasya hy phalaà
goñu yajnah pratiñöhitah
gävo bhaviçyaà bhütaà ca
goñu -yajïäù pratiñöhitaù
“It is the cow which awards the result of sacrifice. The performance of sacrifice is dependent upon her. She is the past and future. All sacrifices are based upon her.”

säyaà prätaçca satataà
homa käle mahädyute
gävo dadati vai homyam
åñibhyaù puruñarsabha
“O greatly powerful King, every morning and evening when the sages perform fire sacrifices, it is the cow who supplies them the essential ingredients, in the form of ghee, etc.”

yäni käni ca durgäni
duñkåtäni kåtäni ca
taranti caiva papmänaà
dhenuà ye dadati prabho
“My dear King, those who give milk-cows in charity become liberated from all types of danger, and also become free from all sinful reactions.”

ekäm ca dañugurdadyät
daça dadyätcca goçati
çatam sahasra purdadyät
sarve tulya phalä hy te
“One who has ten cows should donate one cow, one who has one hundred cows should donate ten cows, and one who has one thousand cows should donate one hundred cows. All these persons are eligible to achieve the same amount of piety.”

anähitägniù satagur
yajvä ca sahasra guù
samåddho yaçca kinaço
närdhyamarhanti te traya
“A person who does not perform agnihotra sacrifices, even after keeping one hundred cows, a person who does not perform fire sacrifices, despite possessing one thousand cows, and a person who does not give up miserliness, are not fit to be shown any respect.’

kapiläà ye prayacchanti
sa vatsäà kämsya dohanäm
subratäà vastra samvitäm
ubhau lokau jayanti te
“A person who donates a brown cow that is full of auspicious symptoms, along with her calf, and also donates a pot made of bell metal for milking the cow, conquers both this world and the next.”

yuvänäm indriyopetäà çatena çata yuöhapaà
gavendram brähmanendräya bhuri çåìgam alaìkåtäà
vrsabhaà ye prayacchanti çrotriyäya paraìtapa
aiçvaryaà te’ dhigacchanti jäya mänäù punaù punaù
“O destroyer of the enemy, those who donate one hundred healthy, strong, nicely decorated bulls, that have strong horns, along with one hundred cows, to a qualified brahmana, who is expert in the knowledge of the Vedas, certainly achieve great wealth in this life and in future lives as well.”

nä kértayitvä gäha supyät
täsäà saàsmrtyä cotpatet
säyäà prätar namasye ca
gästataù pustimäpnuyät
“Do not go to bed at night without praising cows. Do not get up in the morning without remembering the cow. Offer respect to the cow daily, in the morning. By doing so, a human being achieves strength and nourishment.”

gaväà mütra purisasya
nodvijeta kathaïcana
na cäsäà mäàsa maçnéyäd
gaväà pustià tathäpnuyät
“Do not hate cow urine and cow dung. Never eat cow meat. By following this advice, human beings can become prosperous.”

gäçca sankértaye nityaà
näva manyeta tästathä
anistaà svapramälaksya
gäà naraù samprakértayet
“Chant the name of the cow daily and never insult her. If one sees a bad dream, one should immediately remember the cow.”

gomayena sadä snäyät
karése cäpi saàviçot
çlesma mütra purisäni
pratighätaà ca varjayet
“Before taking a daily bath, apply cow dung. Sit down on a place smeared with cow dung. Do not spit, urinate and pass stool upon cow dung. Always avoid hitting cows.”

ghåtena juhuyädagni
ghåtena svasti väcayet
ghåtaà dadyät ghåtaà präçod
gaväà pustià sadäçnute
“Use ghee in fire sacrifices. Use ghee in all auspicious activities. Donate ghee and also use it for personal necessities. By doing this, the human beings will always support the cows and understand their value.

gomatyä vidyayä dhenuà
télänäà abhimantrya yaù
sarva ratra mayià daddyätra
sa çocet kåtäkåte
“A person who donates a cow to a qualified brahmana, along with sesame seed, after duly performing the necessary ritual, does not have to suffer the consequences of his pious and sinful activities.”

gävo maàupatisthantu
hema çrìgyäh payo mucah
surahu saurabhe yaçca
saritah sägaraà yathä
“As the rivers flow into the ocean, may Surabhi and Saurabheyi cows that give milk and have horn covered with gold, come to me.”

gävai paçyäàyahaà nityaà
gävah paçyantu mäà sadä
gävo’ smäkaà vayaà täsäà
yato gävastato vayaà
“May I always see the cow and may the cow always see me. The cow belongs to me and I belong to the cow. I wish to live in a place where cows live.”

evaà rätrau divä cäpi
samesu visamesu ca
mahäbhayeçu can narah
kértayan mucchyate bhayät
“A person who, either during the day or at night, either in happiness or at a time of fearful conditions, remembers or glorifies the cow, certainly becomes freed from all fearful conditions.”

gaväà dåñövä namaskåtya , kuryäcciva pradakñinaà
pradakiñné kåtä tena, saptadvipä vasundharä
mätarah sarva bhütänäà, gävah sarva sukha pradäh
våddhià äkaàçatä nityaà, gävah käryäh pradakñinäh
“One should see, offer obeisances, and circumambulate the cow. By doing so, one is suppose to have circumambulated the entire earth, with its seven islands. The cow is the mother of all. She gives happiness to everyone. People who desire prosperity should daily circumambulate the cow.”