Whole story of Matsya is narrated in Satapatha Brahmana.
1:33. It said, 'As long as we are small, there is great destruction for us: fish devours fish. Thou wilt first keep me in a jar. When I outgrow that, thou wilt dig a pit and keep me in it. When I outgrow that, thou wilt take me down to the sea, for then I shall be beyond destruction.'
1:8:1:44. It soon became a ghasha (a large fish); for that grows largest (of all fish) 3. Thereupon it said, 'In such and such a year that flood will come. Thou shalt then attend to me (i.e. to my advice) by preparing a ship 1; and when the flood has risen thou shalt enter into the ship, and I will save thee from it.'
1:8:1:55. After he had reared it in this way, he took it down to the sea. And in the same year which the fish had indicated to him, he attended to (the advice of the fish) by preparing a ship; and when the flood had risen, he entered into the ship. The fish then swam up to him, and to its horn he tied the rope of the ship, and by that means he 2 passed swiftly up to yonder northern mountain.
1:8:1:66. It then said, 'I have saved thee. Fasten the ship to a tree; but let not the water cut thee off 3, whilst thou art on the mountain. As the water subsides, thou mayest gradually descend!' Accordingly he gradually descended, and hence that (slope) of the northern mountain is called 'Alarm's descent 1.' The flood then swept away all these creatures, and Manu alone remained here.
In Shatpatha Brahmana 7:5.1 & 5 Kurma avatar is mentioned.
- He then puts down a (living) tortoise;--the tortoise means life-sap: it is life-sap (blood) he thus bestows on (Agni). This tortoise is that life-sap of these worlds which flowed away from them when plunged into the waters 1: that (life-sap) he now bestows on (Agni). As far as the life-sap extends, so far the body extends: that (tortoise) thus is these worlds.
5.And as to its being called 'kûrma' (tortoise);Pragâpati, having assumed that form, created living beings. Now what he created, he made; and inasmuch as he made (kar), he is (called) 'kûrma;' and 'kûrma' being (the same as) 'kasyapa' (a tortoise), therefore all creatures are said to be descended from Kasyapa.
In Mahanaryana Upanishad Varaha avatar is mentioned.
bhūmirdhenurdharaṇī lokadhāriṇī . uddhṛtāsi varāheṇa kṛṣṇena śatabāhunā .. 38..
The earth is the giver of happiness like the milk cow, the sustainer of life and support for all living beings. (Represented as such the earth is addressed:)Thou wert raised up by Kṛṣṇa in His incarnation of the boar having hundred hands.