On a baldric he wore a great horn tipped with silver that now was laid upon his knees.

...
Boromir had a long sword, in fashion like Andúril but of less lineage and he bore also a shield and his war-horn.
'Loud and clear it sounds in the valleys of the hills,' he said, `and then let all the foes of Gondor flee!' Putting it to his lips he blew a blast, and the echoes leapt from rock to rock, and all that heard that voice in Rivendell sprang to their feet.

(LoTR, II-3)

Then suddenly with a deep-throated call a great horn blew, and the blasts of it smote the hills and echoed in the hollows, rising in a mighty shout above the roaring of the falls. 'The horn of Boromir!' he cried. 'He is in need!'

(LoTR, II-10)

Boromir had blown his great horn till the woods rang, and at first the Orcs had been dismayed and had drawn back; but when no answer but the echoes came,..

(LoTR, III-3)

'I remember that Boromir bore a horn,' he said at last.
`You remember well, and as one who has in truth seen him,' said Faramir. `Then maybe you can see it in your mind's eye: a great horn of the wild ox of the East, bound with silver, and written with ancient characters. That horn the eldest son of our house has borne for many generations; and it is said that if it be blown at need anywhere within the bounds of Gondor, as the realm was of old, its voice will not pass unheeded. 'Five days ere I set out on this venture, eleven days ago at about this hour of the day, I heard the blowing of that horn: from the northward it seemed, but dim, as if it were but an echo in the mind.

(LoTR, IV-5)

I have received this, said Denethor, and laying down his rod he lifted from his lap the thing that he had been gazing at. In each hand he held up one half of a great horn cloven through the middle: a wild-ox horn bound with silver.
That is the horn that Boromir always wore! cried Pippin.
Verily, said Denethor. And in my turn I bore it, and so did each eldest son of our house, far back into the vanished years before the failing of the kings, since Vorondil father of Mardil hunted the wild kine of Araw in the far fields of Rhûn. I heard it blowing dim upon the northern marches thirteen days ago, and the River brought it to me, broken: it will wind no more. He paused and there was a heavy silence.

(LoTR, V-1)