Hawzah News Agency – The merciful prophet of Islam, peace be upon him and his household says:

خَمسٌ لا أدَعُهُنَّ حَتّى المَماتِ: ... والتَّسليمُ على‏ الصِّبيانِ لِتَكونَ سُنَّةً مِن بَعدي.

'There are five things that I will never leave until death: ... and greeting the young, in order that it may remain a practice after me.'

Source: Bihar al-Anwar, v. 76, p. 10, no. 38