Musannaf Ibn Abi Shaybah

Hazrat Dinar Mawla Atiyyah says that I prayed with Hazrat Qais bin Abad. During the prayer, I held a stick and brought it to my mouth, so he struck my chin. When he completed the prayer, I asked him why he had done such an act that surprised me. He replied that it was said that whoever does any useless act during his prayer, that much is deducted from his prayer.