Fight or Fast (Hadith No. 2476)

Bismillah.

Volume 4, Book 52, Number 81 :
Narrated by Anas bin Malik (radiallaahu `anhu)
In the life-time of the Prophet (sallallaahu `alayhi wasallam), Abu Talha (radiallaahu `anhu) did not fast because of the Jihad, but after the Prophet (sallallaahu `alayhi wasallam) died I never saw him without fasting except on ‘Id-ul-Fitr and ‘Id-ul-Adha.

Prophet (sallallaahu `alayhi wasallam) used to encourage his men to keep up their strength instead of fasting (voluntarily):

تقووا لعدوكم بالإفطار