When To Combine And Shorten Prayers.
Shortening The Prayers Whilst Travelling
Ibn Umar narrates the Messenger of Allaah (Sallalahu Alayhee Wasallam) would never exceed more than 2 rakahs when travelling (ie each prayer) and Abu Bakr (RadhiAllaahu Anhu), Umar (RadhiAllaahu Anhu) and Uthmaan (RadhiAllaahu Anhu) would also do the same. (Bukhaari no.1102, Muslim no.689). Ibn Taymiyyah said the Messenger of Allaah (Sallalahu Alayhee Wasallam) never prayed 4 rakahs whilst a traveller. Imaam Shawkaanee said only shortening of the prayer is proven from the Messenger of Allaah (Sallalahu Alayhee Wasallam) whilst a traveller. (Majmoo’a al-Fataawaa 24:8, As-Sayal al-Jaraar 1:306)
When To Shorten
The opinion which is most popular and has precedence with the scholars is that a journey is when the person considers it a journey and when one establishes he is on a journey he can shorten the prayers. The opinion of a journey is a journey when one thinks he is on a journey is the opinion of Imaams Ibn Taymiyyah, Ibn Qayyim and Shawkaanee. (Majmoo’a al-Fataawaa 24:51, Zaad al-Ma’ad 1:481, as-Sayal al-Jaraar 1:306 and Nayl al-Awtaar 2:480)
How Long Can One Shorten The Prayer.
As long as a person acknowledges he is on a journey and he continues to be on a journey, without the intention of residing at that particular place and he also has the intention of returning back to his city, then according to the opinion of Ibn Taymiyyah he can shorten the prayers. (Majmoo’a al-Fataawaa 24:10)
Nothing with regards to a time period is established from the Messenger of Allaah (Sallalahu Alayhee Wasallam) as to when a person can shorten. This is the case with Messenger of Allaah (Sallalahu Alayhee Wasallam) on the conquest of Makkah and Tabook. The companion Anas Ibn Maalik (RadhiAllaahu Anhu) resided in the area of Persia due to a war expedition and he remained there for 2 years and he continued to shorten his prayer in that time period. (Tahdheeb at-Aathaar Musnad Umar 1:257)
Combining the Prayers
Zuhr can only be combined with Asr and they can be either prayed at the time of Zuhr or Asr. Combining whilst on a journey is proven and established from the Messenger of Allaah (Sallalahu Alayhee Wasallam). (Bukhaari no.1112, Abu Dawood no.1220)