It is narrated from the Mother of the Believers Sayyidah Umm Salamah (may Allah be pleased with her); Shahr bin Hawshab (may Allah have mercy on him) said: I visited her in Madinah, and there was a screen between me and her, and I heard her say: "The supplication the Prophet (peace and blessings be upon him) made most often was: 'Ya muqallib al-qulub, thabbit qalbi 'ala dinik' (O Turner of hearts, keep my heart firm upon Your religion)." Then he (peace and blessings be upon him) said: "There is no human being but that his heart is between two of the fingers of the Most Merciful; when He wills He makes it deviate, and when He wills He guides it."
Hadith Referenceالمعجم الأوسط / باب الألف / 2381
Hadith Takhrij«أخرجه الترمذي فى «جامعه» برقم: 3522، وأحمد فى «مسنده» برقم: 27162، 27219، 27321، والطيالسي فى «مسنده» برقم: 1713، وأبو يعلى فى «مسنده» برقم: 6919، 6986، وعبد بن حميد فى «مسنده» برقم: 1534، وابن أبي شيبة فى «مصنفه» برقم: 29807، 31045، والطبراني فى «الكبير» برقم: 772، 785، 865، والطبراني فى «الأوسط» برقم: 5330، 9432»