How sound is the Hadith: "If you commit a sin, follow it up with a good deed that will erase it," and is the statement "Lā ilāha illallāh (There is no god but Allah)" the best of good deeds, as stated in the Hadith?

The aforementioned hadith was narrated by Imam Ahmad and Al-Tabari from Abu Dharr, and it was graded as hasan (good) by Al-Arna'ut and Al-Albani. In it, the Prophet, peace and blessings be upon him, said to Abu Dharr: "Fear Allah, and if you commit a sin, follow it with a good deed that will erase it." When asked if "La ilaha illallah" (There is no god but Allah) is among good deeds, he replied: "It is among the best of good deeds," and in other narrations: "It is the best of good deeds" or "It is the most beautiful of good deeds."