Srimad Bhagavatam Canto 01, Chapter 05, Text 37

SB 1.5.37

om namo bhagavate tubhyam
vasudevaya dhimahi
namah sankarsanaya ca
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
Let us all chant the glories of Vasudeva along with His plenary expansions Pradyumna, Aniruddha and Sankarsana.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
According to Pañcaratra, Narayana is the primeval cause of all expansions of Godhead. These are Vasudeva, Sankarsana, Pradyumna and Aniruddha. Vasudeva and Sankarsana are on the middle left and right, Pradyumna is on the right of Sankarsana, and Aniruddha is on the left of Vasudeva, and thus the four Deities are situated. They are known as the four aides-de-camp of Lord Sri Krsna.
This is a Vedic hymn or mantra beginning with omkara pranava, and thus the mantra is established by the transcendental chanting process, namely, om namo dhimahi, etc.
The purport is that any transaction, either in the field of fruitive work or in empiric philosophy, which is not ultimately aimed at transcendental realization of the Supreme Lord, is considered to be useless. Naradaji has therefore explained the nature of unalloyed devotional service by his personal experience in the development of intimacy between the Lord and the living entity by a gradual process of progressive devotional activities. Such a progressive march of transcendental devotion for the Lord culminates in the attainment of loving service of the Lord, which is called prema in different transcendental variegatedness called rasas (tastes). Such devotional service is also executed in mixed forms, namely mixed with fruitive work or empiric philosophical speculations.
Now the question which was raised by the great rsis headed by Saunaka regarding the confidential part of Suta’s achievement through the spiritual master is explained herein by the chanting of this hymn consisting of thirty-three letters. And this mantra is addressed to the four Deities, or the Lord with His plenary expansions. The central figure is Lord Sri Krsna because the plenary portions are His aides-de-camp. The most confidential part of the instruction is that one should always chant and remember the glories of the Lord Sri Krsna, the Supreme Personality of Godhead, along with His different plenary portions expanded as Vasudeva, Sankarsana, Pradyumna and Aniruddha. These expansions are the original Deities for all other truths, namely either visnu-tattva or sakti-tattvas.
Srimad Bhagavatam Canto 01, Chapter 05, Text 36
Srimad Bhagavatam Canto 01, Chapter 05, Text 38