Tafsir Ibn Kathir -> Surah Az-Zukhruf -> Ibrahim's Declaration of Tawhid Here

Allah tells us about His servant, Messenger and close Friend, the leader of the monotheists and the father of all subsequent Prophets, from whom Quraysh were descended and claimed to have taken their religion. He disowned his father's and his people's worship of idols and said:

﴿وَإِذْ قَالَ إِبْرَهِيمُ لاًّبِيهِ وَقَوْمِهِ إِنَّنِى بَرَآءٌ مِّمَّا تَعْبُدُونَ - إِلاَّ الَّذِى فَطَرَنِى فَإِنَّهُ سَيَهْدِينِ وَجَعَلَهَا كَلِمَةً بَـقِيَةً فِى عَقِبِهِ﴾

("Verily, I am innocent of what you worship, except Him Who created me; and verily, He will guide me.'' And he made it a Word lasting among his offspring,) The Word means the worship of Allah Alone with no partner or associate, and the denunciation of all gods apart from Him, i.e., La ilaha illallah. He left this word as an example to be followed by those of his progeny who were guided by Allah.

﴿لَعَلَّهُمْ يَرْجِعُونَ﴾

(that they may turn back.) means, come back to this word. `Ikrimah, Mujahid, Ad-Dahhak, Qatadah, As-Suddi and others commented on the Ayah:

﴿وَجَعَلَهَا كَلِمَةً بَـقِيَةً فِى عَقِبِهِ﴾

(And he made it a Word lasting among his offspring,) "This means, La ilaha illallah, and there are still those among his offspring who say it.'' A similar view was narrated from Ibn `Abbas, may Allah be pleased with him. Ibn Zayd said, "The word of Islam,'' which refers to the same thing suggested by the group.