Sahih Muslim 838
Sahih Muslim · Book 6: The Book of Prayer - Travellers · Between every two calls, there is a prayer
This hadith has been narrated by Abdullah b. Mughaffal by another chain of transmitters, but with this variation that he (the Holy Prophet) said at the fourth time:
وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، حَدَّثَنَا عَبْدُ الأَعْلَى ، عَنِ الْجُرَيْرِيِّ ، عَنْ عَبْدِ اللَّهِ، بْنِ بُرَيْدَةَ عَنْ عَبْدِ اللَّهِ بْنِ مُغَفَّلٍ ، عَنِ النَّبِيِّ صلى الله عليه وسلم . مِثْلَهُ إِلاَّ أَنَّهُ قَالَ فِي الرَّابِعَةِ " لِمَنْ شَاءَ " .
" He who wishes (may do to)."