Jaabir b. Abdillaah – may Allaah be pleased with father and son – said: ‘I heard the Messenger of Allaah – sallallaahu alahyi wa sallaam saying: ‘The best word of remembrance of Allaah is:
Laa ilaaha illa Llaah
[There is none worthy of worship except Allaah].
Recorded by at-Tirmidhee and he said it is hasan. Ash-Shaykh Saleem al-Hilaalee – hafidhahullaah – accepted it so in Saheehul-Adh’kaar p.24