Musnad Ahmad 30:139

Sufyan narrated to us, from al-Zuhri, from Sa’id, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said—and it was said to him once: "Did you attribute it [to the Prophet]?" He said: "Yes." And he said another time: "He reaches with it [to the Prophet]": "They say: 'Al-Karam' (generosity/nobility), but indeed, 'Al-Karam' is the heart of the believer."