Hadees related to Dhikr (Zikar)

Jabir ibne- 'Abdullah Radiyallahu 'anhuma narrates: I heard Rasulullah Sallallahu 'alaihi wasallam saying: The best Dhikr (remembrance of ALLAH) is La ilaha illallah, and the best Du'a (supplication) is Alhamdulillah (Praise be to ALLAH). (Tirmidhi)

Note: This Kalimah is the basis of the entire Deen. Without the belief in Kalimah, neither Iman (faith) nor Aa'mal (deeds) are acceptable. Alhamdulillah (Praise be to ALLAH) is said to be the best Du'a because praising ALLAH, Who is the Most Generous, amounts to asking Him for His help or favours. (Mazahir Haque)