Al-Kāfi
Hadith 14563ـ وَبِهَذَا الإِسْنَادِ فِي رَجُلَيْنِ اخْتَلَفَا فَقَالَ أَحَدُهُمَا كُنْتُ إِمَامَكَ وَقَالَ الآخَرُ أَنَا كُنْتُ إِمَامَكَ فَقَالَ صَلاَتُهُمَا تَامَّةٌ قُلْتُ فَإِنْ قَالَ كُلُّ وَاحِدٍ مِنْهُمَا كُنْتُ أَئْتَمُّ بِكَ قَالَ صَلاَتُهُمَا فَاسِدَةٌ وَلْيَسْتَأْنِفَا.
3. Through the chain of narrators as that of the previous Hadith is the following narration: “If two people disagree and one says, ‘I was your Imam’ and the other says, ‘I was your Imam’ he (the Imam) said, ‘Both persons’ Salah (prayer) is complete.’ I (the narrator) then asked, ‘What happens if each one says, ‘I followed you.’ He (the Imam) said, ‘Both persons’ Salah (prayer) is invalid. They must perform it again.’”