I know they had bows for hunting but I've never seen or heard a source that talks about vikings using bows in warfare, why not?
While much more can always be said, in short, they in fact did. I quoted some relevant sources before in: Were archers seen as cowardly by the vikings and other germanic as well as celtic cultures?
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I've never seen or heard a source that talks about vikings using bows in warfare, why not?
In addition to the contemporary texts cited in the linked thread above, I can quote a famous scene of battle in later saga traditions. The following (translation) is taken from Snorri's Heimskringla version of the saga of Olav Tryggvason, depicting the battle of Svolder :
"Einarr þambarskelfir was on Ormrinn aft in the krapparúm (‘narrow space’, a position aft of the fyrirrúm). He was shooting from a bow and he was the strongest shooter of all men. Einarr shot at Jarl Eiríkr and struck the neck of the tiller above the jarl’s head, and the arrow sank in right up to the arrow-head fastening. The jarl looked at it and asked if they knew who was shooting there, but immediately another arrow came, so close to the jarl that it flew between his side and his arm and then into the head-board behind so that the point went through a long way.
Then the jarl spoke to the man that some call Finnr, though some say that he was Finnish (Lappish)—he was the finest bowman:
‘Shoot that big man in the krapparúm.’
Finnr shot, and the arrow struck the middle of Einarr’s bow at the moment that Einarr was drawing his bow for the third time. Then the bow broke in two pieces. Then said King Óláfr:
‘What was it snapped there so loud?’
Einarr replied: ‘Norway from your grasp, king.’
‘It was not as big a snap as that,’ says the king; ‘take my bow and shoot with it.’
And he threw the bow to him. Einarr took the bow and immediately drew it past the point of the arrow and said:
‘Too weak, too weak the supreme ruler’s bow,’ and threw the bow back,
then took his shield and sword and fought (Óláfs saga Tryggvasonar, Chap. 108, in [Finlay & Faulkes trans. 2016: 226f.]."
When the popular edition of Heimskringla was published in Norway in the late 19th century, this scene is also attached with the illustration by Christian Krohg.
The archer in question, Einarr þambarskelfir (modern Norwegian: Einar Tambarskjelve) (d. 1050) was a Norwegian chieftain with well attested historicity, and also one of the key figures in Norway who played a crucial role in politics in the first half of the 11th century.
Another version of the saga of Olav Tryggvason, authored by Oddr Snorrason the Monk, also narrates the alleged use of bow by the protagonist of the saga, King Olav Tryggvason of Norway also in the battle of Svolder, by the mouth of one of his followers, as following:
"Kolbjörn reported as the king was shooting during the day, blood was running out from under his byrnie sleeve. But when looked at the jarl's company that was boarding the ship, he became rather apprehensive and ran to the place the king had been standing. When he did not see him, he jumped overboard into the sea...... (Óláfs saga Tryggvasonar, Chap. 67 (75). The translation is taken from: [Andersson trans. 2003: 133]."
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