This mantra/sloka is found in the Yajurveda.

शिवाय विष्णु रूपाय शिव रूपाय विष्णवे |
शिवस्य हृदयं विष्णुं विष्णोश्च हृदयं शिवः ||
यथा शिवमयो विष्णुरेवं विष्णुमयः शिवः |
यथाsन्तरम् न पश्यामि तथा में स्वस्तिरायुषि|
यथाsन्तरं न भेदा: स्यु: शिवराघवयोस्तथा||

This mantra/sloka is found in the Yajurveda. It can also be found towards the end of the Yajurveda Sandhyavanam.

Shivaaya Vishnu Roopaaya, Shiva Roopaaya Vishanave |
Shivasya Hrudayam Vishnur, Vishnuscha Hrudayam Shivaha ||
Yatha Shivamayo Vishnuhu, Yevam Vishnu Mayaha Shivaha |

Yathaantharam Na Paschyaami, Thatha Me Swasthi Ra Yushi ||

Meaning :
Shivaya vishnu roopaya- vishnu's roopa/beauty and his avatar is none other than shiva. In other words: shiva is vishnu.
the one in shiva roopa is none other than vishnu. In other words: vishnu is shiva.
vishnu resides in shiva's heart, and shiva resides in vishnu's heart.
vishnu will be found in the same place you find shiva, and shiva can be found in the same place vishnu is. As is if you find one, you have found the other.
we are one and the same. Until a person dont find any difference between those two he will be safe and long living"

*************************************
सकंदपुराण २३ । ४१ :

यथा शिवस्तथा विष्णुर्यथा विष्णुस्तथा शिव: ।
अन्तरं शिवविष्ण्वोश्र भनागपि न विद्यते ।।
अर्थ = "जैसे शिव हैं, वैसे ही विष्णु हैं तथा जैसे विष्णु हैं, वैसे हीशिव हैं । शिव और विष्णु में तनिक भी अंतर नहीं है।"

AGNIPURANA :
yo vishnu bhakthivyajena, shiva bhakthichhalena vaa |
dvestinaa shankaram vishnum, tham gruhnidvam mamanthikam ||

Meaning: bring them to me "people who hates vishnu in the name of shiva and people who hates shiva in the name of vishnu" said yama.

Jai Shri Shankar Narayana ~ Jai Shri Hariharaya Namah 🐚👁🌀🕉💀🔱🙏
Shubh Prabhat 🙏❤️🙏