Musnad Ahmad 26:624

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, he said: Mansur wrote to me, and I read it to him, he said: Ibrahim narrated to me, from ‘Alqamah, from ‘Abdullah, who said: The Messenger of Allah (ﷺ) performed a prayer—I do not know whether he added or omitted [something]—Ibrahim is the one who says: "He does not know," [or] ‘Alqamah said: "He added or omitted," or ‘Abdullah [said it]—then he turned to face us, and we told him what he had done. So he folded his leg, faced the Qiblah, and performed two prostrations. Then he turned his face toward us and said: "If something had occurred in the prayer, I would have informed you of it. But I am only a human being; I forget just as you forget. So if I forget, remind me. And whoever among you is in doubt regarding his prayer, let him strive for what is closest to the correct [action], complete it upon that, say the taslim, and then perform two prostrations."