The Messenger of Allah ﷺ said, "Whenever a servant says 'There is no god but Allah' with sincerity, the gates of heaven are opened for it until it reaches the Throne, so long as he avoids the major sins."
قال رسول الله ﷺ: ما قال عبد لا إله إلا الله قط مخلصا إلا فتحت له أبواب السماء حتى تفضي إلى العرش ما اجتنب الكبائر.
Tirmidhi (3590) and Nasai in Amal al-Yawm wal-Layla (833) both narrated it from Husayn bin Ali bin Yazid al-Sudai al-Baghdadi, who said, Walid bin Qasim bin Walid al-Hamdani narrated to us, from Yazid bin Kaysan (he is al-Yashkuri), from Abu Hazim (he is Sulayman al-Ashjai), from Abu Hurairah who said, and then he related it.Its chain of transmission is hasan because of Husayn bin Ali al-Sudai, Walid bin Qasim, and Yazid bin Kaysan, for they are narrators whose hadith are hasan. Tirmidhi said, "This is a hasan gharib hadith from this route."A note of caution, the mention of Abu Hazim is missing from the printed edition of Amal al-Yawm wal-Layla, but it is confirmed in Sunan al-Kubra (10601).