Usama b. Zaid (Allah be pleased with him) reported that he sat behind Allah's Messenger (ﷺ) on his ride as he came back from 'Arafa. And as he came to the valley, he halted his camel, and then went to the wilderness (to urinate). And when he came back, I poured water on him from the jug and he performed ablution, and then rode on until he came to Muzdalifa and there he combined the sunset and 'Isha' prayers.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ، أَخْبَرَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، عَنْ عَطَاءٍ، مَوْلَى سِبَاعٍ عَنْ أُسَامَةَ بْنِ زَيْدٍ، أَنَّهُ كَانَ رَدِيفَ رَسُولِ اللَّهِ صلى الله عليه وسلم حِينَ أَفَاضَ مِنْ عَرَفَةَ فَلَمَّا جَاءَ الشِّعْبَ أَنَاخَ رَاحِلَتَهُ ثُمَّ ذَهَبَ إِلَى الْغَائِطِ فَلَمَّا رَجَعَ صَبَبْتُ عَلَيْهِ مِنَ الإِدَاوَةِ فَتَوَضَّأَ ثُمَّ رَكِبَ ثُمَّ أَتَى الْمُزْدَلِفَةَ فَجَمَعَ بِهَا بَيْنَ الْمَغْرِبِ وَالْعِشَاءِ .
sunnah.com