Do Shia believe 12 imams are better than Ibrahim, what's is their proof from Shia sources?
Shias believe the that the 12 imams are better than all prophets except Muhammad. They use the following verse as evidence:
"And when Abraham was tested by his Lord with certain commands and he fulfilled them. Then He said: Lo! I appoint you an Imam for mankind.”(Qur’an 2:124).
وَإِذِ ابْتَلَىٰ إِبْرَاهِيمَ رَبُّهُ بِكَلِمَاتٍ فَأَتَمَّهُنَّ قَالَ إِنِّي جَاعِلُكَ لِلنَّاسِ إِمَامًا قَالَ وَمِن ذُرِّيَّتِي قَالَ لَا يَنَالُ عَهْدِي الظَّالِمِينَ
From al-Islam.org(a Shia-based website):
As we can see, Prophet Ibrahim was further tested by Allah during his prophethood, and when he successfully passed the tests (which were the test on his life, leaving his wife, sacrificing his son), he was granted the position of Imamat. This shows position of Imamat is higher in degree than prophethood which has been given to him later after acquiring more qualifications.
Also, Allamah Tabatabai interprets this verse like this:
“Towards the end of his life, after passing many hard exams, Prophet Ibrahim (pbuh) reached the level of imamate, although he had it before, but at the same time didn’t bear the capacity necessary for it.”
(Seyyid Mohammad Baqir Musavi Hamedani, Translation of Tafsir Al-Mizan, vol. 1, pg. 405.)
From what I gather, Ibrahim was one of the closest Prophets to imamate according to Shias but they believe that the imams have achieved much more.
These are claims that I found from youpuncturedtheark.wordpress:
Shia Muhaddith al-Hurr al-`Amili has a chapter in his book “al-Fusoul al-Muhimmah fi Usoul al-A’imah” vol.1 pg.403 called: “chapter 101: The Prophet and the twelve Imams (as) are better than all other creations from prophets and past successors and angels, and that the prophets are better than the angels.” Then he mentioned two narrations in it to prove this.
Ibn Babaweih al-Saduq mentioned in his book “`Uyoun Akhbar al-Rida” vol.1 pg.262 a chapter called: “The superiority of the prophet and the Imams over all angels and prophets (as).”
al-Sayyed Amir Muhammad Kadhim al-Qazwini says in his book “al-Shia fi `Aqa’idihim wa Ahkamihim” mentions the title of this chapter on pg.73: [The Imams of Ahlul-Bayt (as) are better than the prophet (as) according to the text of the Qur’an]
To summarise up, Shias believe that the 12 imams and the Prophet are superior to all other prophets and angels therefore they are better than Ibrahim but they regard Ibrahim as one of the closest to the power of the Imamate that they possessed.
They believe that the imams know more than the other prophets because Muhammad knew the most and passed it down to the imams so the other prophets wouldn't have had that knowledge. This is an example from islamquest.net:
“The Great Name” of Allah (swt) consists of seventy three “letters” in which Asef ibn Barqiya knew of one, Prophet Isa (pbuh) knew of two, Prophet Musa (pbuh) knew of four, Prophet Ibrahim (pbuh) knew of eight, Prophet Nuh (pbuh) knew of fifteen, Prophet Adam (pbuh) knew of twenty five and Prophet Muhammad (pbuh) knew of seventy two of them in which he passed on to the imams after him.
And Allah knows best.