Sunan Ibn Majah
3812
Sahih+3

It was narrated from Abu Hurairah that :the Messenger of Allah (ﷺ) said: "Whoever says: Subhan Allahi wa bi hamdihi (Glory and praise is to Allah) one hundred times, his sins will be forgiven even if they were like the foam of the sea

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi