One who desires his ultimate self-interest should cultivate friendship with those persons who have accepted Kṛṣṇa as the Lord of their life. One should further develop an attitude of service toward all living beings. One should especially try to help those in the human form of life and, among them, especially those who accept the principles of religious behavior. Among religious persons, one should especially render service to the pure devotees of the Supreme Personality of Godhead.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
एवं कृष्णात्मनाथेषु मनुष्येषु च सौहृदम् । परिचर्यां चोभयत्र महत्सु नृषु साधुषु
evaṃ kṛṣṇātmanātheṣu manuṣyeṣu ca sauhṛdam | paricaryāṃ cobhayatra mahatsu nṛṣu sādhuṣu
SB 11.3.29