Sahih al-Bukhari 2924
Fighting for the Cause of Allah (Jihaad)
The fighting against Ar-Rum (the Byzantines)
Narrated Khalid bin Madan: That 'Umair bin Al-Aswad Al-Anasi told him that he went to 'Ubada bin As-Samit while he was staying in his house at the sea-shore of Hims with (his wife) Um Haram. 'Umair said. Um Haram informed us that she heard the Prophet (ﷺ) saying, "Paradise is granted to the first batch of my followers who will undertake a naval expedition." Um Haram added, I said, 'O Allah's Messenger (ﷺ) ! Will I be amongst them?' He replied, 'You are amongst them.' The Prophet (ﷺ) then said, 'The first army amongst' my followers who will invade Caesar's City will be forgiven their sins.' I asked, 'Will I be one of them, O Allah's Messenger (ﷺ) ?' He replied in the negative."