SB 6.15.27
sri-narada uvaca
etam mantropanisadam
praticcha prayato mama
yam dharayan sapta-ratrad
drasta sankarsanam vibhum
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The great sage Narada continued: My dear King, attentively receive from me a mantra, which is most auspicious. After accepting it from me, in seven nights you will be able to see the Lord face to face.