From 'The Fellowship of the Ring'
"Elladan and Elrohir were out upon errantry: for they rode often far afield with the Rangers of the North, forgetting never their mother's torment in the dens of the orcs." (FotR, Many Meetings)
"The sons of Elrond, Elladan and Elrohir, were the last to return; they had made a great journey, passing down the Silverlode into a strange country, but of their errand they would not speak to any save to Elrond." (FotR, The Ring Goes South)
From 'The Return of the King'
"Elrohir said to him (Aragorn), 'I bring word to you from my father: The days are short. If thou art in haste, remember the Paths of the Dead.'
'Always my days have seemed to me too short to achieve my desire,' answered Aragorn. 'But great indeed will be my haste ere I take that road.'
'That will soon be seen,' said Elrohir. 'But let us speak no more of these things upon the open road.'" (RotK, The Passing of the Grey Company)
"And have you marked the brethren Elladan and Elrohir? Less sombre is their gear than the others', and they are fair and gallant as Elven-lords; and that is not to be wondered at in the sons of Elrond of Rivendell.'" (RotK, The Passing of the Grey Company)
"...two tall men, neither young nor old. So much alike were they, the sons of Elrond, that few could tell them apart; dark-haired, grey-eyed, and their faces elven fair, clad alike in bright mail beneath cloaks of silver grey." (RotK, The Passing of the Grey Company)
"...for no mightier men had she (Eowyn) seen than the Dúnedain and the fair sons of Elrond." (RotK, The Passing of the Grey Company)
"Behind them (Legolas and Gimli) rode Elladan, last of the Company, but not the last of those that took the downward road... 'Yes, the Dead ride behind. They have been summoned,' said Elladan." (RotK, The Passing of the Grey Company)
"...and Elladan and Elrohir with stars on their brow..." (RotK, The Battle of the Pelennor Fields)
"And Aragorn arose and went out, and he sent for the sons of Elrond, and together they laboured far into the night" (RotK, The Houses of Healing)
"Then said Elrohir, 'From the North we came with this purpose, and from Elrond our father we brought this very counsel. We will not turn back.'" (RotK, The Last Debate)
"But in the front towards Mordor where the first bitter assault would come there stood the sons of Elrond on the left with the Dúnedain about them" (RotK, The Black Gate Opens)