Sahih al-Bukhari Book 78, Hadith 29
A bedouin came to the Prophet (ﷺ) and said, "You (people) kiss the boys! We don't kiss them." The
Prophet said, "I cannot put mercy in your heart after Allah has taken it away from it."
Allah's Messenger (ﷺ) kissed Al-Hasan bin `Ali while Al-Aqra' bin H`Abis at-Tamim was sitting beside him. Al-Aqra said, "I have ten children and I have never kissed anyone of them," Allah's Messenger (ﷺ) cast a look at him and said, "Whoever is not merciful to others will not be treated mercifully."
Some Sabi (i.e. war prisoners, children and woman only) were brought before the Prophet (ﷺ) and behold, a woman amongst them was milking her breasts to feed and whenever she found a child amongst the captives, she took it over her chest and nursed it (she had lost her child but later she found him) the Prophet said to us, "Do you think that this lady can throw her son in the fire?" We replied, "No, if she has the power not to throw it (in the fire)." The Prophet (ﷺ) then said, "Allah is more merciful to His slaves than this lady to her son."