Abu Sa'eed Al-Khudri narrated that the Messenger of Allah (S.A.W) said:
"When one of you visits the ill, then reassure him regarding his lifespan. Indeed that will not repel anything, but it will comfort his soul." (FR)
English reference (FR): Vol. 4, Book 2, Hadith 2087 (FR)
Arabic reference (FR): Book 28, Hadith 2231 (FR)