Musnad Ahmad 528:2

Al-Walid ibn Muslim narrated to us, he said: I heard—meaning Ibn Jabir—saying: Busr ibn Ubayd Allah al-Hadrami narrated to me that he heard Abu Idris al-Khawlani saying: I heard al-Nawwas ibn Sam'an al-Kilabi saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "There is no heart except that it is between two fingers of the fingers of the Lord of the Worlds. If He wills to make it upright, He makes it upright, and if He wills to cause it to deviate, He causes it to deviate." And he used to say: "O Turner of hearts, make our hearts firm upon Your religion. And the Scale is in the hand of the Most Merciful; He lowers it and He raises it."