The sincere disciple of the pure devotee considers the spiritual master equal to the Lord, but always considers himself to be a humble servant of the servant of the Lord. This is the pure devotional path.
(Srimad Bhagavatam---1:1:15---purport).

No one can be the benefactor of the Lord; everyone is actually the servant of the servant of the Lord.
(Srimad Bhagavatam---3:16:20---purport).

A devotee of the Lord always considers himself a servant of the servant of the servant of the Lord.
(Srimad Bhagavatam---5:1:20---purport).

One must accept a spiritual master who comes in the disciplic succession and is a servant of the servant of the Lord. Under his direction, one must then engage one's three properties, namely his body, mind and words.
(Srimad Bhagavatam---6:11:24---purport).

Fear of material existence is due to false ego. Therefore the ultimate goal of life for every living entity is to attain the position of being servant of the servant of the Lord.
(Srimad Bhagavatam---7:9---summary).

One should not be anxious to offer direct service to the Lord. Śrī Caitanya Mahāprabhu advised that one become a servant of the servant of the servant of the Lord. This is the process for approaching the Supreme Lord.
(Srimad Bhagavatam---7:9:28---purport).

The Supreme Personality of Godhead, is very favorable to devotees who act according to the instructions of His own pure devotee. In other words, one must become the servant of the servant of the servant of the Lord. If one wants to become the servant of the Lord directly, this is not as fruitful as engaging in the service of the Lord's servant.
(Srimad Bhagavatam---7:9:42---purport).

According to the Gauḍīya-Vaiṣṇava philosophy of Caitanya Mahāprabhu, it is better to be a servant of the servant of God.
(Sri Caitanya Caritamrta---2:5:23---purport).

In the preaching work of the Kṛṣṇa consciousness movement, we, as the servant of the servant of the servant of the servant of the Supreme Personality of Godhead, fully believe in the words of Kṛṣṇa and His servants, the disciplic succession.
(Sri Caitanya Caritamrta---2:5:76---purport).

A Kṛṣṇa conscious person knows that he is always the servant of the servant of Kṛṣṇa; he is most insignificant, and whatever good he does is by the grace of Kṛṣṇa and the spiritual master.
(Krsna Book).