Musannaf Ibn Abi Shaybah

Hazrat Abu Thamamah Qamah says that at the place of Balat, I met Hazrat Ka'b. I interlaced my fingers, so he struck my hand forcefully and said that we have been forbidden from cracking our fingers in prayer. I said to him, "May Allah have mercy on you! Are you seeing me in prayer?" He said, "Whoever performs ablution and sets out with the intention of prayer is considered to be in prayer."