So far as I know (he was cauterising) their ears.
Sahih Muslim Book 37, Hadith 166
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Anas, see that nothing is given to this child until he is brought to Allah's Apostle (ﷺ) in the morning, so that he should chew some dates and touch his palate with it. I went to him in the morning and he was in the garden at that time having the mantle of Jauniyya over him and he was bus in cauterising (the camels) which had been brought to him (as spoils of war) in victory (over the enemy).
Sahih Muslim Book 37, Hadith 165 Next Hadith → We went to Allah's Messenger (ﷺ) as he was in the fold and he was cauterising the animals of the flock and I think (he was cauterising them) on their ears. This hadith has been narrated on the authority of Shu'ba through another chain of transmitters.
Sahih Muslim Book 37, Hadith 167
Sahih Muslim Book 37, Hadith 165 Next Hadith → We went to Allah's Messenger (ﷺ) as he was in the fold and he was cauterising the animals of the flock and I think (he was cauterising them) on their ears. This hadith has been narrated on the authority of Shu'ba through another chain of transmitters.
Sahih Muslim Book 37, Hadith 167