It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever says 'La ilaha illallah' (none has the right to be worshipped except Allah) before everything, and says 'La ilaha illallah' after everything, is kept safe from worry and grief."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10645