Hadithcore
Mishkat al-Masabih, 2340

Abu Bakr as-Siddiq reported God's messenger as saying, “He who asks pardon is not a confirmed sinner, even if he returns to his sin seventy times a day.” Tirmidhi and Abu Dawud transmitted it.

وَعَنْ أَبِي بَكْرٍ الصِّدِّيقِ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَا أَصَرَّ مَنِ اسْتَغْفَرَ وَإِنْ عَادَ فِي الْيَوْمِ سَبْعِينَ مَرَّةً» . رَوَاهُ التِّرْمِذِيُّ وَأَبُو دَاوُدَ

References1 variant
In-Book Reference
Book 9, Hadith 113