Mishkat al-Masabih 1453
Anas said that God’s Messenger sacrificed with his own hand two horned rams which were white with black markings, mentioned God’s name and said, "God is most great.” He added:
I saw him placing his foot on their sides and saying, "In the name of God. God is most great.” 

(Bukhari and Muslim.)
عَن أنس قَالَ: ضَحَّى رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بِكَبْشَيْنِ أَمْلَحَيْنِ أَقْرَنَيْنِ ذَبَحَهُمَا بِيَدِهِ وَسَمَّى وَكبر قَالَ: رَأَيْته وضاعا قَدَمَهُ عَلَى صِفَاحِهِمَا وَيَقُولُ: «بِسْمِ اللَّهِ وَاللَّهُ أكبر»