After every movement in prayer we say Allahu Akbar. But in I'thidal we say Subhana Rabbial Adheem. Why? One scholar told me that Siddique (may Allah's Mercy be on him) once when he came for prayer was late and was about to miss the 1st Raka'h but he soon joined the Prophet (peace be on him) in Rukoo and that is why Prophet (peace be upon him) said "Rabbana Lakal hamd) which means may Allaah bless who praises him. Is this correct?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The mention of Allaah that is said when standing up from Rukoo' (bowing down) position is Sami'a Allaahu liman hamidah (May Allaah answer he who praises Him), and when in a standing position one says: "Rabbanaa wa lakal-hamd" (Our Lord, for You is all Praise). The evidence about this is the narration of Abu Hurayrah who said: "The Prophet used to start the prayer by saying "Allaahu Akbar" (Allaah is the Greatest), and he again says "Allaahu Akbar" when bowing down. When standing up from Rukoo', he says Sami'a Allaahu liman hamidah", and when in a standing position [after raising from Rukoo'], he says "Rabbanaa wa lakalhamd"…. [Al-Bukhari and Muslim] In another narration the Prophet said: "When the Imaam says ''Sami'a Allaahu liman hamidah", you should say: "Rabbanaa wa lakal-hamd". The reason we say this (mention of Allaah) when we stand up from Rukoo' is because the Prophet used to say so and he ordered us to pray in the same way he prayed. ["Pray as you see me praying"] [Al-Bukhari] The incident that the questioner mentioned is not correct and no books of Sunnah made mention of this. However, the authentic narration that is reported by Imaam Al-Bukhari is that it was Abu Bakrah and not Abu Bakr . Abu Bakrah who came late, quickened his steps and joined the prayer with the Prophet while in Rukoo' position, and it is not mentioned in the narration that the Prophet said this because Abu Bakrah joined the prayer and did not miss Rukoo'.   

Allaah knows best.