The Prophet ﷺ said, "Whoever says when he goes to his bed, 'I seek the forgiveness of Allah the Magnificent, there is no god but He, the Ever-Living, the Sustainer of all, and I turn to Him in repentance,' three times, Allah will forgive him his sins even if they were like the foam of the sea, even if they were the number of the leaves of the trees, even if they were the number of the grains of sand of Alij, and even if they were the number of the days of this world."
حدثنا صالح بن عبد الله حدثنا أبو معاوية عن الوصافي عن عطية
عن النبي ﷺ قال من قال حين يأوي إلى فراشه أستغفر الله العظيم الذي لا إله إلا هو الحي القيوم وأتوب إليه ثلاث مرات غفر الله له ذنوبه وإن كانت مثل زبد البحر وإن كانت عدد ورق الشجر وإن كانت عدد رمل عالج وإن كانت عدد أيام الدنيا
[Its isnad is daif] Ahmad recorded it in 10/3 from Abu Muawiya Dharir with the same wording, and he stated explicitly that he heard it directly. Isam bin Qudama narrated it from Atiya, as found in Bukhari's Tarikh, and Atiya Awfi is daif and a mudallis.