Yes The three gunas is mentioned in Maitri upanishad here
In the beginning 5 darkness (tamas) alone was this. It was in the Highest, and, moved by the Highest, it becomes uneven Thus it becomes obscurity ragas) . Then this obscurity, being moved, becomes uneven. Thus it becomes goodness sattva). Then this goodness, being moved, the essence flowed forth . This is that part (or state of Self) which is entirely intelligent, reflected in man (as the sun is in different vessels of water) knowing the body (kshetragña), attested by his conceiving, willing, and believing, it is Pragâpati, called Visva. His manifestations have been declared before 3. Now that part of him which belongs to darkness(Tamas), that, O students , is he who is called Rudra. That part of him which belongs to Obsucurity(rajas) , that, students, is he who is called Brahmâ. That part of him which belongs to Goodness(sattva) , that, O students, is he who is called Vishnu.He being one, becomes three, becomes eight , becomes eleven , becomes twelve, becomes infinite. Because he thus came to be, he is the Being (neut.), he moves about, having entered all beings, he has become the Lord of all beings. He is the Self within and without, yes, within and without
This gunas are mentioned in maitri upanishad I don't think these are mentioned in samhitas but I will look for refernces.
The words goodness,absucurity,Goodness means rajasnTamas,sattva in sanskrit.
Sanskrit verse can be found in last verse of forth prapathaka here instead of beginning of fifth prapathaka as sacred-texts translation says.
तमो वा इदमेकमास तत्पश्चात्परेणेरितं विषयत्वं
प्रयात्येतद्वै रजसो रूपं तद्रजः खल्वीरितं विषमत्वं
प्रयात्येतद्वै तमसो रूपं तत्तमः खल्वीरितं तमसः
सम्प्रास्रवत्येतद्वै सत्त्वस्य रूपं तत्सत्त्वमेवेरितं
प्रतिपुरुषं क्षेत्रज्ञः सङ्कल्पाध्यवसायाभिमानलिङ्गः
प्रजापतिस्तस्य प्रोक्ता अग्र्यास्तनवो ब्रह्मा रुद्रो विष्णुरित्यथ
यो ह खलु वावास्य राजसोंऽशोऽसौ स योऽयं ब्रह्माथ यो ह
खलु वावास्य तामसोंऽशोऽसौ स योऽयं रुद्रोऽथ यो ह
खलु वावास्य सात्विकोंऽशोऽसौ स एवं विष्णुः स वा एष
चोद्भूत उद्भूतत्वाद्भूतेषु चरति प्रतिष्ठा
ह्च ॥ ५॥ चतुर्थः प्रपाठकः ॥