Riyad as-Salihin

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 (ﷺ) 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].