29 He now proceeds to the divine blessings which were conferred upon them as they wandered in the desert. “He spread out a cloud to be their covering: and fire to give them light in the night season”. This is as clear as it is well known.
30. “They asked, and the quail came”. They did not desire quails, but flesh. But since the quail is flesh, and in this Psalm he speaks not of the provocation of those who did not please God, but of the faith of the elect, the true seed of Abraham; they are to be understood to have desired that that might come which might crush the murmurs of those who provoked. Then in the next line, “And He filled them with the bread of heaven,” he has not indeed named manna, but it is obscure to none who has read those records.
31. “He opened the rock of stone, and the waters flowed out: so that rivers ran in the dry places”. This fact too is understood as soon as read.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)