Sahih Muslim , Book 4, Hadith 1477 (USC-MSA) ⓘ

Salim b. 'Abdullah (b. 'Umar) reported on the authority of his father that Allah's Messenger observed the prayer of a traveller, i. e. two rak'ahs in Mina, and other places; so did Abu Bakr and 'Umar, and 'Uthman too observed two rak'ahs at the beginning of his caliphate, but he then completed four.

وَحَدَّثَنِي حَرْمَلَةُ بْنُ يَحْيَى، حَدَّثَنَا ابْنُ وَهْبٍ، أَخْبَرَنِي 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 ‏.‏