Commentary Par V 122-123
The poetic playfulness of the canto, so evident near its beginning and at its end (see the second part of [ C.Par.V.16-18]), is present here as well, both in the rima composta 'Dì, dì' (Say, say) and in the rapidly repeated sounds of di in these verses ('Dì, dì… credi come a dii… t'annidi"). Beatrice excitedly urges Dante (whose name happens to begin with that sound) on in his increasing hunger for knowledge of heavenly things. (For an even more exhilarated passage, see [Par VII 10-12].)

Why all these repetitions in the concluding verses of the canto? Here dì dì (pronounced, in order to rhyme with annidi and ridi, 'dìdi'), and then in verse 138: chiusa chiusa, and in 139: canto canta? Does the device of anaphora (repetition) have a thematic purpose, mirroring things that can be represented only by themselves (as is the case with vows)? And we may also note that every rhyme word in [Par V 121-127] ends in 'i'; for the same phenomenon, see also [Inf XIII 55-61]; [Inf XVIII 13-19]; and [Inf XXIV 136-142], etc.