The Holy Prophet thereafter commissioned Ali to proceed to Yemen. A force of three hundred soldiers assembled at Quba outside Madina. The Holy Prophet tied the turban on the head of Ali with his own hands, and handed him the standard. The Holy Prophet prayed to God to enlighten the mind of Ali, to make his tongue eloquent, to make his talk impressive, and to make his conduct a source of attraction for others. Although Ali proceeded to Yemen at the head of a force, he was commanded not to use force. The force was only meant for defense in the case of any attack. It was a voyage of enlightenment. People were to be converted to Islam not by an appeal to arms but by an appeal to their hearts.
"When a person dies, his works end, except for three: ongoing charity, knowledge that is benefited from, and a righteous child who prays for him."
Prophet Mohammed (PBUH)
"The best of what a man leaves behind are three: a righteous child who supplicates for him, ongoing charity the reward of which reaches him, and knowledge that is acted upon after him."
Sunan Ibn Mājah
"Every day two angels come down from Heaven and one of them says, 'O Allah! Compensate every person who spends in Your Cause,' and the other (angel) says, 'O Allah! Destroy every miser.'"
Sahih Bukhari