SB 11.18.17

maunānīhānilāyāmā
 daṇḍā vāg-deha-cetasām
na hy ete yasya santy aṅga
 veṇubhir na bhaved yatiḥ
Synonyms: 
mauna — avoiding useless speech; anīha — giving up fruitive activities; anila-āyāmāḥ — controlling the breathing process; daṇḍāḥ — strict disciplines; vāk — of the voice; deha — of the body; cetasām — of the mind; na — not; hi — indeed; ete — these disciplines; yasya — of whom; santi — exist; aṅga — My dear Uddhava; veṇubhiḥ — by bamboo rods; na — never; bhavet — is; yatiḥ — a real sannyāsī.
Translation: 
One who has not accepted the three internal disciplines of avoiding useless speech, avoiding useless activities and controlling the life air can never be considered a sannyāsī merely because of his carrying bamboo rods.
Purport: 

The word daṇḍa indicates the staff carried by those in the renounced order of life, and daṇḍa also indicates severe discipline. The Vaiṣṇava sannyāsīs accept a staff made of three bamboo rods, signifying dedication of the body, mind and words to the service of the Supreme Lord. Here Lord Kṛṣṇa says that one must first accept these three daṇḍas, or disciplines (namely control of the voice, body and mind), within oneself. The practice of anilāyāma (or prāṇāyāma, regulating the life air) is meant to control the mind, and one who always thinks of service to Lord Kṛṣṇa has certainly achieved the perfection of prāṇāyāma. Merely carrying the three external daṇḍas without assimilating the internal daṇḍas of bodily, mental and vocal discipline can never make one an actual Vaiṣṇava sannyāsī, as explained here by Lord Kṛṣṇa.

In the Haṁsa-gītā section of Mahābhārata and in Śrīla Rūpa Gosvāmī’s Upadeśāmṛta, there are instructions regarding the sannyāsa order of life. A conditioned soul who adopts only the external ornaments of tridaṇḍi-sannyāsa will not actually be able to control the senses. One who takes sannyāsa for false prestige, making a show of saintliness without actual advancement in kṛṣṇa-kīrtana, will soon be vanquished by the external energy of the Lord.