17. Amīr al-mu’minīn, peace be upon him, was asked to explain the saying of the Messenger of Allāh: “Banish your old age (by hair-dye) and do not acquire resemblance to the Jews.” Amīr al-mu’minīn replied:
The Prophet (S) said this at a time when the religion was confined to a few, but now that its expanse has widened and it is firmly settled everyone is free in his action. 
 The intention is that since in the beginning of Islam the number of Muslims was limited it was necessary to keep them distinct from the Jews in order to maintain their collective entity, so the Prophet ordered the use of hair-dye which was not in use among the Jews. Besides, it was also the aim that when facing the enemy the people should not appear old in age and weak.