Sunan Ibn Majah, Chapter: Chapters Regarding Funerals

"When Qasim the son of the Messenger of Allah (saw) died, Khadijah said: 'O Messenger of Allah, the milk of Qasim's mother is overflowing. Would that Allah had let him live until he had finished breastfeeding.' The Messenger of Allah (saw) said: 'He will complete his breastfeeding in Paradise.' She said: 'If I know that, O Messenger of Allah, it makes it easier for me to bear.' The Messenger of Allah (saw) said: 'If you wish, I will pray to Allah to let you hear his voice.' She said: 'O Messenger of Allah, rather I believe Allah and His Messenger.'" Sahih