It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) led a prayer and became confused (in the recitation) during it. When he finished, he said to Ubayy bin Ka'b (may Allah be pleased with him): "Did you pray with us?" He said: Yes. He (peace and blessings be upon him) said: "Then what stopped you from prompting (reminding) me?"
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13159