From Sahih Muslim (Hadith 3101):
Usama b. Zaid (Allah be pleased with him) narrated:AHah's Messenger (ﷺ) was on his way back from 'Arafat and as he reached the creek (of a hillock) he got down and urinated (Usama did not say that he poured water), but said: He (the Holy Prophet) called for water and performed ablution, but it was not a thorough one. I said: Messenger of Allah, the prayer! Thereupon he said: Prayer awaits you ahead (at Muzdalifa). He then proceeded, until he reached Muzdalifa and observed sunset and 'Isha' prayers (together) there
Reference: Muslim 3101