As per some books that I have, the said Shloka is part of a Guru Stotram. This Stotram is originally from Viswasara Tantram. At present, this text is lost as a whole. Only some portions of it are preserved as quotes in some other texts like Brihat Tantrasara etc.
The Green Message website also confirms the same.
Guru Stotram - In sanskrit with meaning
- from Vishwasara Tantram (also many slokas are found in Guru Gita)
श्री गुरुस्तोत्रम्: गुरुर्ब्रह्मा गुरुर्विष्णुर्गुरुर्देवो महेश्वरः
Sri Guru Stotram: Gurur Brahma Gurur Vishnu Gurudevo Maheshwara
गुरुरेव परं ब्रह्म तस्मै श्रीगुरवे नमः ॥१॥ Gurur-Brahmaa
Gurur-Vissnnur-Gururdevo Maheshvarah |
Gurure[-I]va Param Brahma
Tasmai Shrii-Gurave Namah ||1||
1.1: The Guru is Brahma, the Guru is Vishnu, the Guru Deva is Maheswara (Shiva),
1.2: The Guru is Verily the Para-Brahman (Supreme Brahman); Salutations to that Guru.