Jami` at-Tirmidhi 3587

Asim bin Kulaib Al-Jarmi narrated from his father, from his grandfather, who said:“I entered upon the Prophet (ﷺ) while he was praying and he had placed his left hand on his left thigh, and he had placed his right hand on his right thigh, and clasped his fingers, and extended his index finger, and he was saying: ‘O changer of the hearts, make my heart firm upon Your religion (Yā muqallibal-qulūbi thabbit qalbī `alā dīnik).’”