Canto 2 -
Ayodhyā-kāṇḍa
Chapter 100: Rāma Instructs Bharata About the Duties of a King
Text 2.100.17

मन्त्रो विजयमूलं हि राज्ञां भवति राघव।
सुसंवृतो मन्त्रधरैरमात्यैः शास्त्रकोविदैः॥

mantro vijaya-mūlaṁ hi rājñāṁ bhavati rāghava
susaṁvṛto mantradharair amātyaiḥ śāstra-kovidaiḥ

mantraḥ = advice; vijaya-mūlam = the root cause of victory; hi = indeed; rājñām = for kings; bhavati = is; rāghava = O descendant of Raghu; susaṁvṛtaḥ = the well-guarded; mantradharaiḥ = of such advisory; amātyaiḥ = ministers; śāstra-kovidaiḥ = expert in Artha-śāstra.

O descendant of Raghu, the well-guarded advice of such advisory ministers expert in Artha-śāstra is indeed the root cause of victory for kings.