(62) باب إِذَا صَلَّى لِنَفْسِهِ فَلْيُطَوِّلْ مَا شَاءَ
Narrated Abu Huraira:
Allah's Messenger (ﷺ) 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. "
Reference: Sahih al-Bukhari 703
In-book reference: Book 10, Hadith 98
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 11, Hadith 671