In the Book of Kibrit al-Ahmar and other books, it is stated that Aba al-Fadl al-Abbas (peace be upon him) participated with his father the Commander of the Faithful (peace be upon him) in the war of Siffin and had heroic positions that proved his worth to fight heroes of that time, and since, he has been famous for his valour and bravery.
Among these courageous positions is when Mu'awiya had taken control of the Euphrates and ordered thousands of fighters to not allow the camp of the Imam Ali (peace be upon him) to get to the water, until the army of the Imam (peace be upon him) suffered from thirst, then the Imam (peace be upon him) has delivered a speech to his companions, inciting them to occupy the Euphrates, and ordered the grandson of the Holy Prophet (Allah's prayers be upon him and upon his holy Household); Imam al-Hussayn (peace be upon him) to lead this mission. So Imam al-Hussayn (peace be upon him) with a group of knights, including his brother Aba al-Fadl al-Abbas (peace be upon him) fought the army of Mu'awiyah near the Euphrates and won that battle and then they occupied the Euphrates, and drank from its water but they did not behave same with Mu'awiyah but they allowed his army to drink from the Euphrates' water.
Also, during the battle of Siffin, Aba al-Fadl al-Abbas (peace be upon him) went to fight in duel while covering his face, so people was afraid of him, so Mu'awiyah ordered Aba Sha'tha' [one of the great fighters of his time] to accept the duel, but Aba Sha'tha' was boastful and refused the duel saying: "People of the Levant consider that I am worth of a thousand knight, and hence it is not fitting me to accept his duel, but instead I will send him one of my sons!" And he had seven sons.
And whenever one of his sons started the fight, Aba al-Fadl killed him, until he killed all the man's sons. So Abu al-Sha'tha got very angry, and the hate of this young knight filled his heart, and said: "I will fight him and I will make his parents saddened by his killing."
And so he went out to fight him, but once he approached, Aba al-Fadl al-Abbas (peace be upon him) hit him once and made him join his seven killed sons. Then, the army of Mu'awiyah feared him, and none of them dared to fight him or to accept his duel, so he went back to his headquarters.
As for the army of the Commander of the Faithful (peace be upon him), they were amazed by this young man's bravery, and were eager to know who he was. So when he came back, Imam Ali (peace be upon him) congratulated him and then he removed the veil from his face, and they could all see that it was his son the Moon of Bani Hashem; Aba al-Fadl al-Abbas (peace be upon him).