Shiva Purana
ब्रह्मा विष्णुश्च रुद्रश्च महेश्वरसदाशिवौ ॥ ते हि साक्षाच्छिवस्यैव मूर्तयः श्रुतिविश्रुताः ॥ २१ ॥
They are Brahmā, Viṣṇu, Rudra, Maheśvara and Sadāśiva. They are the images of Śiva himself well known in the Vedas.
english translation
brahmA viSNuzca rudrazca mahezvarasadAzivau ॥ te hi sAkSAcchivasyaiva mUrtayaH zrutivizrutAH ॥ 21 ॥
hk transliteration by Sanscript