Sahih Muslim 2151
Sahih Muslim · Book 38: The Book of Manners and Etiquette · It Is Permissible To Say: "O My Son" To Someone Other Than One's Son, And It Is Recommended To Speak Kindly
Anas b Malik reported that Allah's Messenger (ﷺ) addressed me:
حَدَّثَنَا مُحَمَّدُ بْنُ عُبَيْدٍ الْغُبَرِيُّ ، حَدَّثَنَا أَبُو عَوَانَةَ ، عَنْ أَبِي عُثْمَانَ ، عَنْ أَنَسِ بْنِ، مَالِكٍ قَالَ قَالَ لِي رَسُولُ اللَّهِ صلى الله عليه وسلم " يَا بُنَىَّ " .
O My Son.