Mishkat al-Masabih 890
Maimuna said that when the Prophet prostrated himself he kept his arms so far away from his sides that if a lamb had wanted to pass under his arms it could have done so. 

This is Abu Dawud’s wording, as [Baghawi] explained in Sharh as-sunna with his isnad. Muslim has something to the same effect:
She said that when the Prophet prostrated himself, if a lamb had wanted to pass between his arms it could have done so.
وَعَن مَيْمُونَة قَالَتْ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا سَجَدَ جَافَى بَيْنَ يَدَيْهِ حَتَّى لَوْ أَنَّ بَهْمَةً أَرَادَتْ أَنْ تَمُرَّ تَحْتَ يَدَيْهِ مرت. هَذَا لفظ أبي دَاوُد كَمَا صَرَّحَ فِي شَرْحِ السُّنَّةِ بِإِسْنَادِهِ وَلِمُسْلِمٍ بِمَعْنَاهُ: قَالَتْ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا سجد لوشاءت بهمة أَن تمر بَين يَدَيْهِ لمرت