Sahih Al Bukhari 1: Chapter 3, Hadith 93

(93) Whoever knelt down before the Imam or a (religious) preacher

حَدَّثَنَا أَبُو الْيَمَانِ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ أَخْبَرَنَا شُعَيْبٌ، ‏‏‏‏‏‏عَنِ الزُّهْرِيِّ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ أَخْبَرَنِي أَنَسُ بْنُ مَالِكٍ، ‏‏‏‏‏‏"أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ خَرَجَ، ‏‏‏‏‏‏فَقَامَ عَبْدُ اللَّهِ بْنُ حُذَافَةَ، ‏‏‏‏‏‏فَقَالَ:‏‏‏‏ مَنْ أَبِي؟ فَقَالَ:‏‏‏‏ أَبُوكَ حُذَافَةُ، ‏‏‏‏‏‏ثُمَّ أَكْثَرَ أَنْ يَقُولَ سَلُونِي، ‏‏‏‏‏‏فَبَرَكَ عُمَرُ عَلَى رُكْبَتَيْهِ، ‏‏‏‏‏‏فَقَالَ:‏‏‏‏ رَضِينَا بِاللَّهِ رَبًّا وَبِالْإِسْلَامِ دِينًا وَبِمُحَمَّدٍ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ نَبِيًّا"، ‏‏‏‏‏‏فَسَكَتَ.

SAHIH HADITH:

Narrated Anas bin Malik: One day Allah's Apostle came out (before the people) and `Abdullah bin Hudhafa stood up and asked (him) Who is my father? The Prophet replied, Your father is Hudhafa. The Prophet told them repeatedly (in anger) to ask him anything they liked. `Umar knelt down before the Prophet and said thrice, We accept Allah as (our) Lord and Islam as (our) religion and Muhammad as (our) Prophet. After that the Prophet became silent.

REFERENCE:

Sahih Al Bukhari 1: Chapter 3, Hadith 93