This hadith has been narrated by Ja'far b. Rabi' with the same chain of transmitters. And in the narration transmitted by 'Amr b. al-Harith (the words are):

" When the Messenger of Allah (rtiay peace be upon him) prostrated, he spread out his arms so that the whiteness of his armpits was visible." And in the narration transmitted by al-Laith (the words are:" When the Messenger of Allah (ﷺ) prostrated. he spread his hands from the armpits so that I saw their whiteness."

حَدَّثَنَا عَمْرُو بْنُ سَوَّادٍ، أَخْبَرَنَا عَبْدُ اللَّهِ بْنُ وَهْبٍ، أَخْبَرَنَا عَمْرُو بْنُ الْحَارِثِ، وَاللَّيْثُ بْنُ سَعْدٍ، كِلاَهُمَا عَنْ جَعْفَرِ بْنِ رَبِيعَةَ، بِهَذَا الإِسْنَادِ ‏.‏ وَفِي رِوَايَةِ عَمْرِو بْنِ الْحَارِثِ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا سَجَدَ يُجَنِّحُ فِي سُجُودِهِ حَتَّى يُرَى وَضَحُ إِبْطَيْهِ ‏.‏ وَفِي رِوَايَةِ اللَّيْثِ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ إِذَا سَجَدَ فَرَّجَ يَدَيْهِ عَنْ إِبْطَيْهِ حَتَّى إِنِّي لأَرَى بَيَاضَ إِبْطَيْهِ ‏.‏