Musannaf Ibn Abi Shayba 3:4874

Hussayn ibn Ali narrated to us, from Za'idah, from Mansur, from Abu al-Duha, from Masruq, who said: Abdullah said: "Observe the prayer with *qaru*." Za'idah said: So I said to Mansur, "What does he mean by that?" He said: He said, "Steadiness (at-tamakkun) in it."