Jami' al-Saghir / Jam' al-Jawami' (al-Suyuti) 18228a
"لَيسَ عَدُوُّكَ الَّذِي يَقْتُلُكَ فَيُدْخِلُكَ الله به الْجَنَّةَ وَإِنْ قَتَلْتَه كَانَ لَكَ نُورًا، وَلَكِنْ أعْدَى الأَعداءِ لَكَ نَفَسُك الَّتي بَينَ جَنبَيك".
Translation
Your enemy is not the one who kills you and thereby Allah admits you into Paradise, and if you kill him it will be a light for you; rather, the most hostile of enemies to you is your own self that lies between your sides.
Notes
العسكرى في الأمثال عن سعيد بن أبي هلال مرسلًا
Grades by grader
No grade is recorded for this wording. Ask what scholars say about its grade