Undoubtedly, Ali ibn Abi Talib is a guardian appointed by Allah. However, the adhan is a specific act of worship legislated by Allah and His Messenger, and it is not permissible for anyone to add something to it, even if it is true in and of itself; as one of our companions informed us, he said:

«سَأَلْتُ الْمَنْصُورَ عَنْ قَوْلِ الشِّيعَةِ فِي الْأَذَانِ: ”أَشْهَدُ أَنَّ عَلِيًّا وَلِيُّ اللَّهِ“، فَقَالَ: بِدْعَةٌ، ثُمَّ قَالَ: إِنِّي وَاللَّهِ لَأَعْجَبُ مِنْ قَوْمٍ يُنَاقِضُونَ أَنْفُسَهُمْ عَلَى كُلِّ مِئْذَنَةٍ! قُلْتُ: وَكَيْفَ ذَلِكَ؟ أَصْلَحَكَ اللَّهُ، قَالَ: يَشْهَدُونَ أَنَّ عَلِيًّا وَلِيُّ اللَّهِ وَهُمْ يُخَالِفُونَهُ فَيُؤَذِّنُونَ بِمَا لَمْ يُؤَذِّنْ بِهِ! وَاللَّهِ مَا كَانَ عَلِيٌّ يَقُولُ هَذَا فِي أَذَانِهِ وَلَا يَأْمُرُ بِهِ!»; “I asked Mansoor about the words of Shias in the adhan: ‘I bear witness that Ali is a guardian (appointed) by Allah.’ He said: ‘It is an innovation.’ Then he said: ‘By Allah, I am amazed at a people who contradict (the claim of) themselves on every minaret!’ I said: ‘How do they do so, may Allah rectify your affair?’ He said: ‘They bear witness that Ali is a guardian (appointed) by Allah, and yet they oppose him, so they say the adhan in a manner that he has not done so! By Allah, Ali did not say this in his adhan, nor did he command it!’”

Therefore, it is not permissible to say «أشهد أنّ عليًّا وليّ اللّه» in the adhan while believing that it is part of the adhan, and if someone says it without believing that it is part of the adhan, then he has spoken during the adhan, which is detested in all Islamic sects, and al-Shafi‘i has considered it recommended to repeat the adhan after it. If someone says it in the iqamah, then he must start over; because the iqamah is like a part of prayer; as Abu Harun al-Makfuf has narrated and said: «قَالَ أَبُو عَبْدِ اللَّهِ -يَعْنِي جَعْفَرَ بْنَ مُحَمَّدٍ الصَّادِقَ- عَلَيْهِ السَّلَامُ: يَا أَبَا هَارُونَ! الْإِقَامَةُ مِنَ الصَّلَاةِ، فَإِذَا أَقَمْتَ فَلَا تَتَكَلَّمْ وَلَا تُؤْمِ بِيَدِكَ»[1]; “Abu Abdullah -that is, Ja‘far ibn Muhammad al-Sadiq- said: ‘O Abu Harun! The iqamah is part of prayer, so when you say it, do not speak or gesture with your hand,’” and Amr ibn Abi Nasr has narrated and said: «قُلْتُ لِأَبِي عَبْدِ اللَّهِ عَلَيْهِ السَّلَامُ: أَيَتَكَلَّمُ الرَّجُلُ فِي الْأَذَانِ؟ قَالَ: لَا بَأْسَ، قُلْتُ: فِي الْإِقَامَةِ؟ قَالَ: لَا»[2]; “I said to Abu Abdullah : ‘Can a person speak during the adhan?’ He said: ‘No problem.’ I said: ‘(What about) during the iqamah?’ He said: ‘No,’” and Muhammad ibn Muslim has narrated and said: «قَالَ أَبُو عَبْدِ اللَّهِ عَلَيْهِ السَّلَامُ: لَا تَتَكَلَّمْ إِذَا أَقَمْتَ لِلصَّلَاةِ، فَإِنَّكَ إِذَا تَكَلَّمْتَ أَعَدْتَ الْإِقَامَةَ»[3]; “Abu Abdullah said: ‘Do not speak when you say the iqamah for prayer, for if you speak, you must repeat the iqamah.’” It is strange that Shias blame Sunnis for saying «الصّلاة خير من النوم»; “Prayer is better than sleep” in the adhan because it has not been narrated in a proven manner, but they themselves say something similar, but rather more baseless! ﴿كَذَلِكَ زُيِّنَ لِلْمُسْرِفِينَ مَا كَانُوا يَعْمَلُونَ﴾[4]; “This is how what the extravagant ones are doing has been made fair-seeming to them!” This is a fact that was not hidden from the early Shia scholars; as Muhammad ibn Ali ibn Babawayh (d. 381 AH), known as Shaikh al-Saduq, has explicitly said in his book Man La Yahduruh al-Faqih[5] that this statement in adhan was one of the innovations of “Mufawwidah, may Allah curse them,” who posed themselves as a group of Shias!