Ibn Rajab:
If the slave of Allaah perfects his Tawheed and his sincerity
for Allaah and he establishes/fulfills all its conditions with his heart, and
his tongue and his limbs then forgiveness for his past sins becomes obligatory.
‘Jama’ al-Aloom’ 2/417
Translation by Ustaadh Abbas Abu Yahya
)hafithahullaah)
Taken from: https://followingthesunnah.com
