Qur’an:

“God said: “Do not take for yourselves two gods. He is but one God, so fear Me!” [An-Nahl 16:51]

وقال الله لا تتخذوا إلهين اثنين إنما هو إله واحد فإياي فارهبون

Sunnah:

Abu Basir reported that Imam As-Sadiq (a.) said about the verse: “Do not take two gods for yourselves. He is only one God.” (16:51): “It is meant by this, that you should not take two Imams for yourselves, for there is only one Imam (at each time).” [Bihar-ul-Anwar of Al-Majlisi, Volume 23 page 357 Hadith 9]

عن محمد بن مسعود عن أبي بصير عن الصادق عليه السلام أنه قال: ولا تتخذوا إلهين اثنين إنما هو إله واحد يعني بذلك ولا تتخذوا إمامين إنما هو إمام واحد

Ni’matullah Al-Jaza’iri writes: “As-Saduq, God have mercy on him, took part in the discussions with the scholars of the majority in the meetings of some kings and when they said: “We have together with you one God and one Prophet and we differ only in the appointment of the first successor.” He (Saduq) said: “What you are claiming, is not the case. Rather there is a wide difference between us and you and that even concerning God and the Prophet, for you claim, that you have a Lord, Who sent a Messenger whose rightful successor is Abu Bakr and we say, that that Lord is not our Lord and we do not declare ourselves in favour of the Prophethood of that Prophet. Rather we say, that our Lord is He, Who appointed for His Messenger (s.) Ali Ibn Abi Talib (a.) as his successor. So where is the agreement?” [Nur-ul-Barahin, Vol. 1 page 59]

قال نعمة الله الجزائري: قال الصدوق تغمده الله برحمته في تمام ما حكيناه عنه في المباحثة مع علماء الجمهور في مجلس بعض الملوك – لما قالوا له: اننا وأنتم على اله واحد ونبي واحد، وافترقنا في تعيين الخليفة الأول ليس الحال على ما تزعمون بل نحن وأنتم في طرف من الخلاف، حتى في الله سبحانه والنبي، وذلك أنكم تزعمون أن لكم ربا، وذلك الرب أرسل رسولا خليفته بالاستحقاق أبو بكر، ونحن نقول: إن ذلك الرب ليس ربا لنا، وذلك النبي لا نقول بنبوته، بل نقول: إن ربنا الذي نص على أن خليفة رسوله علي بن أبي طالب عليه السلام فأين الاتفاق

Comment:

To take both Abu Bakr and Ali (a.) as Imam means to take two Gods, because one God appointed Abu Bakr and the other appointed Ali (a.) and both are not consistent with a non-contradictory God.