Fifth Mantra of the Yajurveda –

Agne vratapate vratam carisyami tacchakeyam tanme radhyatam.
Idamahamanrtat satyamupaimi

The original mantra in sanskrit –

अग्ने व्रतपते व्रतं चरिष्यामि तच् छकेयं तन् मे राध्यताम् ।
इदम् अहम् अनृतात् सत्यम् उपैमि ॥

Meaning of the mantra –

1. There is a controller fire in the cosmos, it is lord or owner of laws.

2. It is the controller of vows, the vows of faith.

Subject of the mantra – 

Diety – Agni Devata,
Rishi –  Parameshthi Prajapati,
Chhanda – Anushtup
Swara – Gandhara