Musannaf Ibn Abi Shayba 33:361

Abu Mu'awiyah narrated to us, from al-A'mash, from 'Amr ibn Murrah, from Salim ibn Abi al-Ja'd, from Umm al-Darda', she said: It was said to her, "What was the best deed of Abu al-Darda'?" She said: "Contemplation (al-tafakkur)."