Musnad Ahmad 338:3

Ya'qub ibn Ibrahim narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: He narrated to me regarding the spreading of the left thigh of the Messenger of Allah, peace and blessings of Allah be upon him, in the middle of the prayer and at its end, and his sitting upon his left hip, and his placing of his left hand upon his left thigh, and his setting upright of his right foot, and his placing of his right hand upon his right thigh, and his setting upright of his index finger, declaring the oneness of his Lord, the Almighty and Majestic, with it. 'Imran ibn Abi Anas, brother of the Banu 'Amir ibn Lu'ayy—and he was trustworthy—narrated from Abu al-Qasim Miqsam, the freed slave of 'Abdullah ibn al-Harith ibn Nawfal, who said: A man from the people of Medina narrated to me, saying: I prayed in the mosque of Banu Ghifar. When I sat in my prayer, I spread my left thigh and set upright my index finger. He said: Khuffaf ibn Ima' ibn Rahdah al-Ghifari—who had companionship with the Messenger of Allah, peace and blessings of Allah be upon him—saw me while I was doing that. He said: When I finished my prayer, he said to me: "O my son, why did you set your finger upright like this?" I said: "What do you disapprove of? I saw the people doing that." He said: "You have done correctly. The Messenger of Allah, peace and blessings of Allah be upon him, used to do that when he prayed." The polytheists used to say: "Muhammad only does this with his finger to cast a spell with it." But they lied. The Messenger of Allah, peace and blessings of Allah be upon him, only used to do that to declare the oneness of his Lord, the Almighty and Majestic, with it.