Musnad Ahmad 30:812

Yazid narrated to us, Hisham bin Hassan informed us, from Muhammad bin Sirin, from Abu Hurairah, who said: "He forbade *al-ikhtisar* (placing the hands on the waist) during prayer." We said to Hisham: "Did he attribute it to the Prophet, may Allah bless him and grant him peace?" He nodded his head, meaning: "Yes."