(3) Caste or Jaati. In Hinduism, there are four Varn - caste or Jaati, in Hindu society - Braahman, Kshatriya, Vaishya and Shoodra. Duties for all of them are fixed and scriptures insist that one should perform his duties (Dharm) according to his Varn.

Origin of Varn
See also   Varn Aashram in Ved
Varn have been originated from the earliest times, some references are given here -

The First is Ved
Rig Ved, Mandal No 10, Sookt No. 90; Verses No 11-16 --

V.11 - Various elements came out from Viraat Purush., which became his Hands, Legs, Thighs, and Mouth.
V.12 - His mouth become Braahman, His arms/hands become Kshatriya, His Thighs are Vaishya, His legs/feet are Shoodra
V.13 - From his mind came out Moon, From his eyes came out Sun, From his mouth came out Indra and Agni, and From his Praan (heart) came out Vaayu (Air)
V.14 - From his head came out Heaven, From his naval came out Antariksh (Sky), and From his legs Earth came out.

This is the Sookt from Rig Ved where Braahman, Kshatriya, Vaishya and Shoodra were mentioned for the first time in Vaidik literature.

The Next is Manu Smriti
Its 1st Chapter, Verses 9-15 --

V.9 - God has created Braahman from his mouth, Kshatriya from his arms, Vaishya from his thighs, and Shoodra from his feet.
V.10 - Education, fire sacrifice (Yagya), to accept donation and also to give donation are the duties of Braahman.
V.11 - Defending of people, to make donation, to perform Yagya through Braahman, Reading of Ved, and to be away from Vaishya (property/money) are the Duties of Kshatriya.
V.12 - Vaishya are to take care of cattle, to offer donation, to perform Yagya, to Study Ved, to do cultivation and business etc.
V.13 - Only one duty is assigned to Shoodra that is to serve Braahman, Kshatriya and Vaishya with all sincerity and devotion.
V.14 - In this Manu Smriti, the details of duties, rules and regulations, work, job etc have been described.
V.15 - Real Dharm is to adhere to the rules and regulations as defined in Shruti and Smriti. Hence everybody must do their work according to their caste for his own benefits.

The Next Comes Bhagavad Geetaa:
Verses - 16:23-24
V. 23 - Ignoring the precept of scriptures (Ved and Smriti), he who acts under the impulsion of passion, He does not attain perfection, or happiness, or the supreme Goal.
V. 24 - Therefore, the scripture (Veda, Smrities) is your authority as regards the determination of what is to be done and what is not to be done. After understanding (your) duty as presented by scriptural injunction, you ought to perform (your duty as Kshatriya) here.

Verses 18:7, 23, 45, 47, and 48
V. 7 - The abandoning of daily obligatory acts is not justifiable. Giving up that through Delusion is declared to be based on "Tamas".
V. 23 - The daily obligatory action which is performed without attachment and without likes or dislikes by one who does not hanker for rewards, that is said to be born of "Sattwa".
V. 45 - Being devoted to his own duties, man attains complete success.
V. 47 - One's own duty (though defective), is superior to another's duty well performed. By performing a duty as dictated by one's own nature, one does not incur sin.
V. 48 - O son of Kuntee, one should not give up the duty to which one is born, even though it is faulty.