question #3 Sahih al-Bukhari 1089, Book 18, Hadith 10 : What are the conditions under which a Muslim is allowed to shorten their prayers?

offered four rak`at of Zuhr prayer with the Prophet (p.b.u.h) at Medina and two rak`at at Dhul-Hulaifa.
(i.e. shortened the `Asr prayer).