Ishavasya Upanishad of Shukla Yajurveda (actaully it's the last part of the Samhita itself) has the following verse-15:
Quoting with Adi Shankaracharya's Bhashya from Wisdom Library:
हिरण्मयेन पात्रेण सत्यस्यापिहितं मुखम् ।
तत् त्वं पूषन्न् अपावृणु सत्यधर्माय दृष्टये ॥ १५ ॥
hiraṇmayena pātreṇa satyasyāpihitaṃ mukham |
tat tvaṃ pūṣann apāvṛṇu satyadharmāya dṛṣṭaye || 15 ||
15. Truths face is covered with a golden lid;
remove that, O Pūṣan, that I, Truth’s devotee, may see It.
Hiranmayam =seeming golden, resplendent tena =by such. pātrena =lid, as it were, satyasya i.e., of the Brahman residing in the Solar disc, apihitam =covered. mukham= entrance. tat= (that); tvam =(you); he-pūṣan =O Sun, apāvṛṇu =remove. satyadharmāya i.e., to me who am through meditation on you who are Truth. Or this expression may mean “one that practises true piety” Dṛṣṭaye i.e., for reaching you whose essence is Truth.
As you can see the verse says Mukha of Satya (Truths face) is covered with a Hiranyamaya Patra (golden lid) and requesting Pushan to remove/open that Patra so-that I obtain the Satya!
This looks very mysterious and interesting spiritual preaching. I want to know more what is the face of truth and what is the (Hiranyaaya) Patra (mentioned in the verse) by which the face of truth is covered/hidden and one whose approach is Satya (that is called SatyaDharma in the verse) is preached to pray Pushan to open the cover.
Btw, This is the title song of one very useful show on Upanishads published in the form of TV Serial named Upanishad Ganga: (I watched all the 52 episodes but this verse is not included/explained in any episode.)
And Adi Shankaracharya's Bhashya doesn't elaborate much on that verse.