taittiriya upnishad chapter -5

bhur, bhuvar, svar , these are the three worlds of his naming. verily the rishi mahachamasya made known a fourth to these, which is mahas. it is brahman, it is the self, and the other devtas are his members.

bhur it is this world, bhuvar, it is the antariksh, svar it is the other world: but mahas is aaditya, by aaditya all these worlds increase and prosper. 

bhur it is agni; bhuvar, it is vaayu, svar, it is aaditya: but mahas is chandrama. by the chandrama, all these shining fires increase and prosper. 

bhur, it is the hymns of the rigveda; bhuvar, it is hymns of the sama, svar, it is the hymns of the yajur: but mahas is the eternal. by the eternal all these vedas increase and prosper. 

bhur it is the main praan; bhuvar it is the lower praan; svar it is the vyaan ,breath pervasor: but mahas is ann. by ann all breaths increase and prosper. 

these are the four and they are fourfold;- four words of his naming and each is four again. he who know these, know the eternal, and to him all devtas carry offering.