Musnad Abu Ya'la al-Mawsili 152:193

Muhammad ibn Abi Bakr narrated to us, Khalid ibn al-Harith narrated to us, Malik ibn Mighwal narrated to us, from Mansur, from Khaythama, from a man, from Abdullah. It was said to him, "Did you hear the Messenger of Allah, may Allah bless him and grant him peace, say: 'Remorse is repentance'?" He said, "Yes." Mujahid said regarding the killing of a soul: "If he feels remorse."