It is narrated from the grandfather of Asim bin Kulayb (may Allah be pleased with him) that he said: I entered the mosque, and the Messenger of Allah (peace and blessings be upon him) had placed his hand on his thigh, pointing with his index finger, and was saying: "Ya muqallib al-qulub, thabbit qalbi 'ala dinik (O Turner of hearts, make my heart firm upon Your religion)."
Hadith Referenceالمعجم الكبير للطبراني / باب الشين / 7201