SB 6.15.27

śrī-nārada uvāca
etāṁ mantropaniṣadaṁ
 pratīccha prayato mama
yāṁ dhārayan sapta-rātrād
 draṣṭā saṅkarṣaṇaṁ vibhum
Synonyms: 
śrī-nāradaḥ uvāca — Śrī Nārada Muni said; etām — this; mantra-upaniṣadam — Upaniṣad in the form of a mantra by which one can achieve the highest goal of life; pratīccha — accept; prayataḥ — with great attention (after finishing the funeral ceremony of your dead son); mama — from me; yām — which; dhārayan — accepting; sapta-rātrāt — after seven nights; draṣṭā — you will see; saṅkarṣaṇam — the Supreme Personality of Godhead, Saṅkarṣaṇa; vibhum — the Lord.
Translation: 
The great sage Nārada 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.