Atharvasiras Upanishad is one of the ancient Upanishad and perhaps the only Upanishad which is quoted in DharmaShastras (like Gautama 19.12, Vasistha 22.9 etc.. ) It is a part of AtharvaVeda. The beginning verse of the 6th chapter of the Upanishad states:
योऽग्नौ रुद्रो योऽप्स्वन्तर्य ओषधीर्वीरुध आविवेश । य इमाविश्वा भुवनानि चक्लृपे तस्मै रुद्राय नमोऽस्त्वग्नये । (Atharvasiras Upanishad 6.1)
Same Rudra who is in the Fire, entered in the waters, in the plants. He created all these worlds (Vishwa-Bhuwana), Salutations to Rudra and his form of Agni.
And the text continues stating few similar verses. Now, this mantra seems to be derivation from mantra of YajurVeda 5.5.9 which is very similar to this mantra:
यो रुद्रो अग्नौ यो अप्सु य ओषधिषु ।
यो रुद्रो विश्वा भुवना विवेश तस्मै रुद्राय नमो अस्तु ।। (YajurVeda 5.5.9)
The Rudra in the fire, in the waters, in the plants, the Rudra that has entered the whole world, to that Rudra be homage.
The difference between above mantra of YajurVeda and the Atharvasiras mantra is that, Yajurveda states:
यो रुद्रो विश्वा भुवना विवेश
Rudra who entered the whole world.
While the Atharvashiras state:
इमाविश्वा भुवनानि चक्लृपे तस्मै
He who created this whole world.
Now, there can be significant philosophical arguments in the both verses. The Yajurveda mantra means 'who entered the whole world.' can also be intrepreted as Brahman just entered the world (which was already there) and it may also seem to support world (Prakirti) doesn't emerge from Brahman and Brahman just enters in the world (like that in Samkhya.. ). While the Atharvasiras mantra says both entered and 'created this whole world' which clearly means world/Prakirti also comes from Brahman and Brahman also enters there.
So Is the Atharvasiras Upanishad just deriving that mantra of YajurVeda or it is quoting from other parts of Veda? It is possible that as Atharvasiras Upanishad belongs to Atharvaveda so it is quoting from Atharvaveda. If it helps the mantra in IAST transliteration is as:
yo'gnau rudro yo'psvantarya oṣadhīrvīrudha āviveśa ।
ya imā viśvā bhuvanāni caklṛpe tasmai rudrāya namo'stvagnaye ।।