Mimamsasutra 42.2.5-3

If it is argued that: “The chant is to be sung upon three verses of either unequal metres or equal metres at will, because there is no determining rule stating ‘only thus’”; / [we reply:] it is not so; because the danger of destruction and deficiency serves as the determining rule. “Destruction” (śara) means injury; “deficiency” (leśa) means smallness; for the two verbal roots are observed: śṝ in the sense of injuring, and liś in the sense of becoming small. If one were to sing a chant that originated upon a base verse of greater metre upon two subsequent verses of smaller metre, then since it would be completed by a mere portion of the chant, the remaining portion of the chant, lacking a support, would be destroyed. If one were to sing it upon two verses whose metre is greater even than that of the base verse, then, because of the smallness of the chant, the remaining portion of the verse would be devoid of chant. Therefore, it must be sung only upon verses of equal metre.

(In the fifth [topical section], in the first interpretation on the phrase “he sings upon the two subsequent verses”: sūtras 23–24):

(In the topical section on the taking of the two verses found in the textual collection:)