Then followed the key sentence denoting the clear designation of 'Ali as the leader of the Muslim ummah. The Prophet [s] held up the hand of 'Ali and said:
"For whoever I am his Leader (mawla), 'Ali is his Leader (mawla)."
[In some narrations the word used was wali rather than mawla - with the same implication]
The Prophet [s] continued:
"O' God, love those who love him, and be hostile to those who are hostile to him."
source : http://www.al-islam.org
