Sahih Muslim Book - 6, Hadith - 1590

وَحَدَّثَنِى حَرْمَلَةُ بْنُ يَحْيَى حَدَّثَنَا ابْنُ وَهْبٍ أَخْبَرَنِى عَمْرٌو وَهُوَ ابْنُ الْحَارِثِ عَنِ ابْنِ شِهَابٍ عَنْ سَالِمِ بْنِ عَبْدِ اللَّهِ عَنْ أَبِيهِ عَنْ رَسُولِ اللَّهِ -صلى الله عليه وسلم- أَنَّهُ صَلَّى صَلاَةَ الْمُسَافِرِ بِمِنًى وَغَيْرِهِ رَكْعَتَيْنِ وَأَبُو بَكْرٍ وَعُمَرُ وَعُثْمَانُ رَكْعَتَيْنِ صَدْرًا مِنْ خِلاَفَتِهِ ثُمَّ أَتَمَّهَا أَرْبَعًا.

It was narrated from Salim bin 'Abdullah, from his father, that the Messenger of Allah (s.a.w) prayed as a traveler in Mina and elsewhere, with two Rak'ah. Abu Bakr and 'Umar also prayed with two Rak'ah, as did 'Uthman at the beginning of his Khilafah (reign), then he completed it and made it four Rak'ah.