Brian Moynihan, The Faith, p. 165.
The desert frontiers of the empire and Persia were undefended because no invaders had come from them before. Khalid ibn al-Walid, the key commander in Iraq and Syria, marched for six days across the waterless country between them. Short of water-skins, he forced twenty camels to drink great quantities of water; then their mouths were tied to stop them from chewing cud that would spoil the water in their stomachs. Each night some of the camels were slaughtered, and the water extracted from their stomachs was enough for the column to reach a spring.