Before migration two Rak'ats were obligatory and after migration four Rak'at were made obligatory. Ayesha (R.A) transmits that ,when Allah enjoined the prayer(during the Night of Ascension),He enjoined it in the form of two Rak'at only both when in residence or in journey.Then after migration the prayers offered in journey remained the same,but the Rak'at of the prayers for the non travellers were increased.(Sahih Muslim). #Salah
