Anas bin Malik reported,
The Messenger of Allah ﷺ used to say often, "O Allah, keep my heart firm upon Your religion."A man asked, "Messenger of Allah, do you fear for us, when we have believed in you and accepted what you have brought?"He replied, "Hearts are between two fingers of the fingers of the Most Merciful 
. He turns them as He wills." And Amash gestured with his two fingers.
Ibn Majah 3834
Hasanحدثنا محمد بن عبد الله بن نمير حدثنا أبي حدثنا الأعمش عن يزيد الرقاشي
عن أنس بن مالك قال
كان رسول الله ﷺ يكثر أن يقول اللهم ثبت قلبي على دينك فقال رجل يا رسول الله تخاف علينا وقد آمنا بك وصدقناك بما جئت به فقال إن القلوب بين إصبعين من أصابع الرحمن عز وجل يقلبها وأشار الأعمش بإصبعيه
[Hasan] Bukhari related it in Adab al-Mufrad, no. 683, from Amash, from Yazid Raqashi and Abu Sufyan, from Anas, with this wording. Tirmidhi, no. 2140, said hasan sahih. It has corroborating narrations, including one related by Tirmidhi, no. 3522, with a hasan chain, from Umm Salama, with this wording, conveying a similar meaning, and he said, this is a hasan hadith.