Skandapurana 1.3.6.64

1.3.6.64

prahasya bhagavānnaṃdī brahmāṇaṃ vai hyuvāca ha..
kva yūyaṃ kva śivaḥ śaṃbhustapasā parameṇa hi..
draṣṭavyo hṛdi madhyasthaḥ so'dya draṣṭuṃ na pāryate.. 8.64 ..

Laughing, the blessed Nandin indeed said to Brahmā: / "Where are you, and where is Śiva Śambhu? Through supreme austerity / he is to be seen residing in the middle of the heart; he cannot be seen today. 8.64