SB 12.6.66

yājñavalkyas tato brahmaṁś
 chandāṁsy adhi gaveṣayan
guror avidyamānāni
 sūpatasthe ’rkam īśvaram
Synonyms: 
yājñavalkyaḥ — Yājñavalkya; tataḥ — thereafter; brahman — O brāhmaṇa; chandāṁsi — mantras; adhi — additional; gaveṣayan — seeking out; guroḥ — to his spiritual master; avidyamānāni — not known; su-upatasthe — he carefully worshiped; arkam — the sun; īśvaram — the powerful controller.
Translation: 
My dear brāhmaṇa Śaunaka, Yājñavalkya then desired to find out new yajur-mantras unknown to even his spiritual master. With this in mind he offered attentive worship to the powerful lord of the sun.