Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.102A.

Abu Hurairah reported that the Prophet, peace be upon him, said, “When a servant of Allah utters the words la ilaha illa’Allah (there is none worthy of worship except Allah) sincerely, the doors of heaven open up for these words until they reach the Throne of Allah, so long as its utterer keeps away from the major sins.” (Tirmizhi, who says it is a hasan gharib hadith)

Abu Hurairah also reported that the Prophet, peace be upon him, said, “Renew your faith.” “How can we renew our faith?” they asked. The Prophet, peace be upon him, replied, “Say always, ‘La ilaha ill-Allah’.”  (Reported by Ahmad with a sound chain of authorities)

Jabir reported that the Prophet, peace be upon him, said, “The best remembrance of Allah is to repeat La ilaha ill-Allah and the best prayer is alhamdu li-Allah (All Praise is due to Allah).”  (Reported by Nasa’i, Ibn Majah, and Al-Hakim, who considers its chain sound)