It is narrated from Sayyiduna Aban al-Muharibi (may Allah be pleased with him) - and he was one of the delegation that came to the Messenger of Allah (peace and blessings be upon him) from Abd al-Qays - that the Messenger of Allah (peace and blessings be upon him) said: "There is no servant who says in the morning: 'All praise is for Allah, my Lord; I associate nothing with Him, and I bear witness that there is no god worthy of worship except Allah,' but he continues to be forgiven throughout the day until evening comes; and if he says it in the evening, he continues to be forgiven throughout the night until morning comes."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 631