The Book of Pilgrimage
حَدَّثَنَا أَبُو كُرَيْبٍ، مُحَمَّدُ بْنُ الْعَلاَءِ حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ سُفْيَانَ، عَنْ مُحَمَّدِ بْنِ، عُقْبَةَ عَنْ كُرَيْبٍ، عَنِ ابْنِ عَبَّاسٍ، قَالَ رَفَعَتِ امْرَأَةٌ صَبِيًّا لَهَا فَقَالَتْ يَا رَسُولَ اللَّهِ أَلِهَذَا حَجٌّ قَالَ " نَعَمْ وَلَكِ أَجْرٌ " .Ibn Abbas (Allah be pleased with them) reported:A woman lifted up her child and said: Messenger of Allah, would the child be credited with having performed the Hajj? Thereupon he said: Yes, and there would be a reward for you
From Sahih Muslim, whose reports the scholars accept as authentic.