The Book of Clothes and Adornment
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ هِشَامِ بْنِ زَيْدٍ، قَالَ سَمِعْتُ أَنَسًا، يُحَدِّثُ أَنَّ أُمَّهُ، حِينَ وَلَدَتِ انْطَلَقُوا بِالصَّبِيِّ إِلَى النَّبِيِّ صلى الله عليه وسلم يُحَنِّكُهُ قَالَ فَإِذَا النَّبِيُّ صلى الله عليه وسلم فِي مِرْبَدٍ يَسِمُ غَنَمًا . قَالَ شُعْبَةُ وَأَكْثَرُ عِلْمِي أَنَّهُ قَالَ فِي آذَانِهَا .Anas reported that when his mother gave birth to a child they brought that child to Allah's Messenger (ﷺ) so that he might chew some dates and touch his palate with them. and Allah's Apostle (ﷺ) was at that time in the fold busy in cauterising the animals Shu'ba said:So far as I know (he was cauterising) their ears
From Sahih Muslim, whose reports the scholars accept as authentic.