Hadees related to Kalima (Kalma)

 Abu Hurairah Radiyallahu 'anhu narrates that Rasulullah Sallallahu 'alaihi wasallam said: Whenever a slave (of Allah) says La ilaha illallah sincerely, the doors of the skies are opened (for it to ascend) until it reaches the 'Arsh (Throne of Allah, and is readily
accepted) provided he abstains from major sins. (Tirmidhi)

Note: To say sincerely means that it is free from pretence and hypocrisy. Abstaining from major sins ensures its rapid acceptance, and even if it is recited without abstaining from the major sins, it is still beneficial and rewarding. (Mirqat-ul-Mafatih)