Bearing witness “laa ilaahah illa allah” and that “muhammadun rasool ullaah” is the first pillar of the five pillars of Islam. And the meaning of “laa ilaahah illa allah” is that nothing worshipped is worthy of worship except Allah—it is simultaneously a denial and affirmation. “laa ilaahah” is denial of all worship other than that of Allah. “illa allaah” is affirmation that all worship is for Allah alone without partners.
As for the declaration, “muhammadun rasool ullaa,” its meaning is the assertion of the message of Mohammed (peace and blessings of Allah be upon him) and to believe in it and to adhere to it by speech, action and faith, and to avoid all that is against it, whether it be sayings, actions, or intentions… In other words, obeying him in what he ordered and believing in what he said and relayed and avoiding what he has forbid and denounced and not worshipping Allah except as he has ordained.
wa-sall-allahu wa-sallim ‘ala nabiyyina muhammadin wa-aalihi wa-sahbih. (May the peace and blessings of Allah be upon our prophet Muhammad and upon his families and companions).
Sheikh Muhammed Salih Al-Munajjid