Mantra 10

येन च्छन्दसा स्तोष्यन्स्यात्तच्छन्द उपधावेद्येन स्तोमेन स्तोष्यमाणः स्यात्तंस्तोममुपधावेत् ॥ १.३.१० ॥

In plain words

Let him take refuge in the metre in which he is about to sing praise, and in the chant with which he is about to praise.