CC Madhya 24.159
karma kāraṇam ucyate
śamaḥ kāraṇam ucyate
ārurukṣoḥ — of a person desiring to rise to the platform of yogic perfection; muneḥ — of a saintly person; yogam — spiritual knowledge; karma — work; kāraṇam — the cause; ucyate — is said; yoga-ārūḍhasya — of one who has attained such perfect knowledge; tasya — for him; eva — certainly; śamaḥ — controlling the mind without being disturbed; kāraṇam — cause; ucyate — is said.
“ ‘For those saintly persons who wish to rise to the platform of yogic perfection, the means consists of practicing the yoga system by strictly following its regulative principles and practicing the yoga postures and breathing exercises. And for those who are already elevated to this platform, the means consists of maintaining mental equilibrium [śama] by rejecting all material activity and practicing meditation to keep the mind on the Supreme Lord.
texts 159 and 160 are from the Bhagavad-gītā (6.3-4).