Al-Tabari reported: Hasan al-Basri, may Allah be pleased with him, said, “Your enemy is not the one you are relieved from if you killed him. Rather, your true enemy is your own soul, between your two sides.”
Al-Tabari said, “It is the jihad against the greatest of enemies to a man. There is no doubt that it is greater in reward to Allah than the jihad against the people of idolatry.”
Source: Tahdhīb al-Āthār Musnad ‘Umar 2/812
عن الطبري قال الحسن البصري رحمه الله لَيْسَ عَدُوُّكَ الَّذِي إِنْ قَتَلْتَهُ اسْتَرَحْتَ مِنْهُ وَلَكِنَّ عَدُوَّكَ نَفْسُكَ الَّتِي بَيْنَ جَنْبَيْكَ
قال الطبري هُوَ جِهَادُ أَعْدَى الْأَعْدَاءِ لِلْمَرْءِ وَذَلِكَ لَا شَكَّ أَعْظَمُ أَجْرًا عِنْدَ اللَّهِ مِنْ جِهَادِ أَهْلِ الشِّرْكِ
2/812 تهذيب الآثار مسند عمر