Where is the word ‘Veda’ written in the Vedas?

The question needs a lot of clarification. If the OP means to ask where is the word “veda” literally found in Vedas, then there are thousands of instances.

The word “#veda” is not only a noun but also a verb. Hence, it occurs quite frequently.

The verb forms come from the dhātu “विद ज्ञाने” i.e. to know. The present tense singular first person and third person (लट् लकार प्रथमपुरुष उत्तमपुरुष एकवचनम्) verb form is veda (वेद).

Just one example out of hundreds in Rig Veda, RV 1.25.7:

वेदा यो वीनां पदमन्तरिक्षेण पतताम् । वेद नावः समुद्रियः ॥

“(Varuna) who knows all the flights of all the birds in the air, and who knows all the ships sailing in the ocean.”

Another famous example in Taittirīya Āraṇyaka 3.12.7 (Purusha Suktam):

वेदाहमेतं पुरुषं महान्तम् — “I know this Great Purusha”

Moreover, the noun “veda” also has multiple meanings in the Vedas. Especially in the Rig Veda Samhita, it means wealth (all kinds — material, spiritual, etc.).

Once again, just one example amongst hundreds, RV 1.99.1:

जातवेदसे सुनवाम सोममरातीयतो निदहाति वेदः । स नः पर्षदति दुर्गाणि विश्वा नावेव सिन्धुं दुरितात्यग्निः ॥

“We offer Soma to Jataveda Agni, who burns up (or may he burn up) the wealth (veda) of evil enemies. He, Agni, safely takes us (or may he safely take us) across all obstacles and difficulties like a ship across the ocean.”

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If the OP means to ask where in the Vedas, the formal Vedas themselves are referenced, even then there a dozens of instances. I have highlighted a few famous instances in an answer to a similar question:

What do the Vedas say about the Vedas?

Another similar answer:

What is the relationship between Rig Veda, Sama Veda and Yajur Veda?

A few more easily-accessible instances that come to memory now:

Chandogya Upanishad 7.2:

ऋग्वेदं भगवोऽध्येमि यजुर्वेदं सामवेदमाथर्वणं चतुर्थम् …

“O Bhagavan, I have studied Rig Veda, Yajur Veda, Sama Veda and Atharva Veda the fourth….”

(This is said by Narada when he approached Sanatkumara for self-realization.)

Mundaka Upanishad 1.1.5:

तत्रापरा ऋग्वेदो यजुर्वेदः सामवेदोऽथर्ववेदः …

“There the aparāvidyā consists of Rig Veda, Yajur Veda, Sama Veda, Atharva Veda….”

Mundaka Upanishad 2.1.6:

तस्मादृचः साम यजूँषि दीक्षा यज्ञाश्च सर्वे क्रतवो दक्षिणाश्च …

“From the Brahman have come the Rks, Samans, Yajuses, the various yajnas, rites and rituals….”

Brhadaranyaka Upanishad 1.2.5:

… इदं सर्वमसृजत यदिदं किंचर्चो यजूँषि सामानि छन्दांसि यज्ञान्प्रजाः पशून् ।

“…. He created all this that exists — Rks, Yajuses, Samans, yajnas, creatures.”

Brhadaranyaka Upanishad 1.5.5:

त्रयो वेदा एत एव वागेवर्ग्वेदो मनो यजुर्वेदः प्राणः सामवेदः ।

“These are verily the three Vedas — speech is Rig Veda, mind is Yajur Veda and prana is Sama Veda.”

Brhadaranyaka Upanishad 2.4.10 & 4.5.11:

स यथार्द्रैधाग्नेरभ्याहितात्पृथग्धूमा विनिश्चरन्त्येवं वा अरेऽस्य महतो भूतस्य निश्वसितमेतद्यदृग्वेदो यजुर्वेदः सामवेदोऽथर्वाङ्गिरसः …

“Just as from a fire lit with wet fuel, many different columns of smoke arise, likewise, from this Great Being’s breath have come Rig Veda, Yajur Veda, Sama Veda, Atharva Veda….”

Brhadaranyaka Upanishad 4.1.2:

… ऋग्वेदो यजुर्वेदः सामवेदोऽथर्वाङ्गिरसः … वाचैव सम्राट् प्रज्ञायन्ते ।

“… Rig Veda, Yajur Veda, Sama Veda, Atharva Veda, … are all known through speech alone, O emperor.” (Yajnavalkya addressing Janaka)

There are dozens more references in the vast Vedic corpus (वेदवाङ्मयम्).