Riyad as-Salihin, Book 18, Hadith 1517 (In-Book) ⓘ

Sufyan bin 'Abdullah (May Allah be pleased with him) reported: I asked: "O Messenger of Allah! Tell me, of something to which I may remain steadfast." He said, "Say: My Rubb is Allah and then remain steadfast." Then I said: "O Messenger of Allah! What do you fear most about me?" He took hold of his own tongue and said: "This."[At-Tirmidhi].

وعن سفيان بن عبد الله رضي الله عنه قال‏:‏ قلت يا رسول الله حدثني بأمر أعتصم به قال‏:‏ ‏ "‏ قل ربي الله ثم استقم‏" ‏ قلت‏:‏ يا رسول الله ما أخوف ما تخاف علي‏؟‏ فأخذ بلسان نفسه، ثم قال‏:‏ “هذا”‏.‏ رواه الترمذي وقال حديث حسن صحيح‏.‏