Call to Prayers (Adhaan)

Bukhari :: Book 1 :: Volume 11 :: Hadith 671

Narrated Abu Huraira:

Allah's Apostle said, "If anyone of you leads the people in the prayer, he should shorten it for amongst them are the weak, the sick and the old; and if anyone among your prays alone then he may prolong (the prayer) as much as he wishes. "