SB 11.18.17
maunanihanilayama
danda vag-deha-cetasam
na hy ete yasya santy anga
venubhir na bhaved yatih
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 sannyasi merely because of his carrying bamboo rods.
Purport:
The word danda indicates the staff carried by those in the renounced order of life, and danda also indicates severe discipline. The Vaisnava sannyasis 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 Krsna says that one must first accept these three dandas, or disciplines (namely control of the voice, body and mind), within oneself. The practice of anilayama (or pranayama, regulating the life air) is meant to control the mind, and one who always thinks of service to Lord Krsna has certainly achieved the perfection of pranayama. Merely carrying the three external dandas without assimilating the internal dandas of bodily, mental and vocal discipline can never make one an actual Vaisnava sannyasi, as explained here by Lord Krsna.
In the Hamsa-gita section of Mahabharata and in Srila Rupa Gosvami’s Upadesamrta, there are instructions regarding the sannyasa order of life. A conditioned soul who adopts only the external ornaments of tridandi-sannyasa will not actually be able to control the senses. One who takes sannyasa for false prestige, making a show of saintliness without actual advancement in krsna-kirtana, will soon be vanquished by the external energy of the Lord.