Salim (may Allah have mercy on him) narrates from his father (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) recited in a certain prayer and became confused in the recitation. When he (peace and blessings be upon him) finished the prayer, he asked 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 prevented you from prompting me?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5783