A woman lifted up a young child of hers to the Messenger of Allah ﷺ and asked, "Messenger of Allah, does this child have a Hajj?"He replied, "Yes, and you will have a reward."
حدثنا محمد بن طريف الكوفي حدثنا أبو معاوية عن محمد بن سوقة عن محمد بن المنكدر
رفعت امرأة صبيا لها إلى رسول الله ﷺ فقالت يا رسول الله ألهذا حج قال نعم ولك أجر
Sourcing, its isnad is sahih. Ibn Majah also narrated it in the Rites, chapter on the pilgrimage of a child, hadith: 2910, from Muhammad bin Tarif, through the same route. Abu Muawiya explicitly stated that he heard it directly. There is also a narration on this subject from Ibn Abbas, Muslim, hadith: 1336.