مُوَطَّأُ مَالِكٍ
Muwatta Malik 850
Hajj
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ إِذَا طَعَنَ فِي سَنَامِ هَدْيِهِ وَهُوَ يُشْعِرُهُ قَالَ بِسْمِ اللَّهِ وَاللَّهُ أَكْبَرُ .
Yahya related to me from Malik from Nafi that Abdullah ibn Umar said, when nicking the hump of his sacrificial animal to brand it, "In the name of Allah, and Allah is greater."
Arabic/English book reference: Book 20, Hadith 147