بُلُوغُ الْمَرَامِ
Bulugh al-Maram 418
The Book of Prayer
وَعَنْ أَنَسٍ قَالَ: { صَلَّى رَسُولُ اَللَّهِ - صلى الله عليه وسلم -فَقُمْتُ وَيَتِيمٌ خَلْفَهُ, وَأُمُّ سُلَيْمٍ خَلْفَنَا. } مُتَّفَقٌ عَلَيْهِ, وَاللَّفْظُ لِلْبُخَارِيّ ِ [1] .
Narrated Anas (RA): Allah's Messenger (ﷺ) prayed and an orphan and I prayed behind him and Umm Sulaim (RA) was behind us. [Agreed upon and the wording is al-Bukhari's].
Sunnah.com reference: Book 2, Hadith 323