Hisn al-Muslim · Book 1 · Hadith 140
مَا مِنْ عَبْدٍ يُذنِبُ ذَنْباً فَيُحْسِنُ الطُّهُورَ، ثُمَّ يَقُومُ فَيُصَلِّي رَكْعَتَيْنِ، ثُمَّ يَسْتَغْفِرُ اللَّهَ إِلاَّ غَفَرَ اللَّهُ لَهُ
There is not any slave of Allah who commits a sin, then he perfects his purification and stands to pray two Rak'ahs of prayer, then seeks Allah's forgiveness, except that Allah will forgive him. Reference: Abu Dawud 2/…