Sahih Bukhari
Call to Prayers (Adhaan)
Volume: 001 / Book Number: 011 / Hadith Number: 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. "