Sahih Bukhari Volume 5, Book 64, Hadith 4291

وَقَالَ اللَّيْثُ حَدَّثَنِي يُونُسُ، قَالَ أَخْبَرَنِي نَافِعٌ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ـ رضى الله عنهما ـ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم أَقْبَلَ يَوْمَ الْفَتْحِ مِنْ أَعْلَى مَكَّةَ عَلَى رَاحِلَتِهِ، مُرْدِفًا أُسَامَةَ بْنَ زَيْدٍ وَمَعَهُ بِلاَلٌ وَمَعَهُ عُثْمَانُ بْنُ طَلْحَةَ، مِنَ الْحَجَبَةِ حَتَّى أَنَاخَ فِي الْمَسْجِدِ، فَأَمَرَهُ أَنْ يَأْتِيَ بِمِفْتَاحِ الْبَيْتِ، فَدَخَلَ رَسُولُ اللَّهِ صلى الله عليه وسلم وَمَعَهُ أُسَامَةُ بْنُ زَيْدٍ وَبِلاَلٌ وَعُثْمَانُ بْنُ طَلْحَةَ، فَمَكَثَ فِيهِ نَهَارًا طَوِيلاً ثُمَّ خَرَجَ، فَاسْتَبَقَ النَّاسُ، فَكَانَ عَبْدُ اللَّهِ بْنُ عُمَرَ أَوَّلَ مَنْ دَخَلَ، فَوَجَدَ بِلاَلاً وَرَاءَ الْبَابِ قَائِمًا، فَسَأَلَهُ أَيْنَ صَلَّى رَسُولُ اللَّهِ صلى الله عليه وسلم فَأَشَارَ لَهُ إِلَى الْمَكَانِ الَّذِي صَلَّى فِيهِ‏.‏ قَالَ عَبْدُ اللَّهِ فَنَسِيتُ أَنْ أَسْأَلَهُ كَمْ صَلَّى مِنْ سَجْدَةٍ

Narrated 'Abdullah bin 'Umar(RA): Allah's Messenger (s.a.w) entered Makkah through its upper part and he was riding his she-camel. Usama bin Zaid was his companion-rider behind him(on the same she-camel). In his company were Bilal and 'Uthman bin Talha, who was one of the Al-Hajabah(who keep the key of the gate of the Kaaba'). When he made his she-camel kneel down in the Mosque(i.e., Al-Masjid-al-Haram), he ordered him (i.e., Uthman) to bring the key of the Kaba'. Then Allah's Messenger(s.a.w) entered the Ka'ba along with Usama bin Zaid, Bilaland Uthman bin Talha, and he stayed in it for a long period and then came out. The people rushed (to get in) and 'Abdullah bin 'Umar was the first to enter and he found Bilal standing behind the door. Ibn 'Umar asked Bilal, "Where did Allah's Messenger(s.a.w) offered the Salat(prayer)? " Bilal showed him the place where he (s.a.w) had offered Salat(prayer). 'Adullah later on said , "I forgot to ask Bilal how many prostrations (i.e., Raka') the Prophet(s.a.w) offered."