Abu Dharr (RAA) narrated that the Messenger of Allah (ﷺ) said:
"Do not consider any act of goodness as being insignificant even if it is meeting your brother with a cheerful face." Related by Muslim. (RU)
English reference (RU): Book 16, Hadith 1506 (RU)
Arabic reference (RU): Book 16, Hadith 1463 (RU)