अथ हाग्नयः समूदिरे तप्तो ब्रह्मचारी कुशलं नः पर्यचारीद्धन्तास्मै प्रब्रवामेति तस्मै होचुः प्राणो ब्रह्म कं ब्रह्म खं ब्रह्मेति ॥ ४.१०.४ ॥
In plain words
Then the fires said among themselves, "The student has practised austerity and has tended us well. Come, let us teach him." And they said to him, "The breath is Brahman, joy is Brahman, space is Brahman."