Śhiva Purana. Section 7.2 - Vāyavīya-saṃhitā (2). Chapter 31 - The Hymn of lord Śiva says that they pray on both Shiva and Shakti.
atha viṣṇurmaheśasya śivasyaiva parā tanuḥ |
vāritattvādhipaḥ sākṣādavyaktapadasaṃsthitaḥ || 126 ||
nirguṇassattvabahulastathaiva guṇakevalaḥ |
avikārābhimānī ca trisādhāraṇavikriyaḥ || 127 ||
asādhāraṇakarmā ca sṛṣṭyādikaraṇātpṛthak |
dakṣiṇāṃgabhavenāpi spardhamānaḥ svayaṃbhuvā || 128 ||
ādyena brahmaṇā sākṣātsṛṣṭaḥ sraṣṭā ca tasya tu |
aṃḍasyāṃtarbahirvartī viṣṇurlokadvayādhipaḥ || 129 ||
asurāṃtakaraścakrī śakrasyāpi tathānujaḥ |
prādurbhūtaśca daśadhā bhṛguśāpacchalādiha || 130 ||
bhūbhāranigrahārthāya svecchayāvātarakṣitau |
aprameyabalo māyī māyayā mohayañjagat || 131 ||
mūrtiṃ kṛtvā mahāviṣṇuṃ sadāśiṣṇumathāpi vā |
vaiṣṇavaiḥ pūjito nityaṃ mūrtitrayamayāsane || 132 ||
śivapriyaḥ śivāsaktaḥ śivapādārcane rataḥ |
śivasyājñāṃ puraskṛtya sa me diśatu maṃgalam || 133 ||
126-133. Viṣṇu is another form of the great lord Śiva himself. He is the lord of the principle of water. He is stationed in the region of the unmanifest. He is Nirguṇa. He is predominantly Sāttvic as well as possessed of single Guṇa. He does not identify himself with the created things. He has the three general aberrations. He is of extraordinary activity apart form that of creation. He compares with Brahmā though he is born of the right part of Śiva. He is created by the first Brahmā and he is his creator too. Viṣṇu abides within and without the universe. He is the lord of the worlds. With his discus he destroys Asuras. He is the younger brother of Indra. He has manifested himself in ten forms under the curse of Bhṛgu. He incarnated on the earth at his will for removing the burden of the Earth. His strength is immeasurable and incomprehensible. He is the wielder of Māyā. He fascinates the universe with his Māyā in the form of Mahāviṣṇu and Sadāviṣṇu. He is worshipped by the devotees of Viṣṇu in the seat of the three Mūrtis. He is a favourite of Śiva and is fondly attached to him. He is engaged in the worship of his feet. May he grant me what is auspicious at the behest of Śiva.
vāsudevo 'niruddhaśca pradyumnaśca tataḥ paraḥ |
saṃkarṣaṇassamākhyātāścatasro mūrtayo hareḥ || 134 ||
matsyaḥ kūrmo varāhaśca nārasiṃho 'tha vāmanaḥ |
rāmatrayaṃ tathā kṛṣṇo viṣṇusturagavaktrakaḥ || 135 ||
cakraṃ nārāyaṇasyāstraṃ pāṃcajanyaṃ ca śārṅgakam |
satkṛtya śivayorājñāṃ maṃgalaṃ pradiśaṃtu me || 136 ||
134-136. The four Mūrtis of Viṣṇu, viz Vāsudeva, Aniruddha, Pradyumna, and Saṃkarṣaṇa, the ten Avataras viz—the fish, tortoise, boar, man-lion, dwarf, Rāma trio, Kṛṣṇa and the horse-faced Kalki, his discus, Nārāyañāstra, Pāñcajanya and the bow Śārṅga—may these grant my desire at the behest of the two (Śiva & Śakti).