It was narrated that Ibn Al-Simt said:
"I saw 'Umar bin Al-Khattab praying two rak'ahs in Dhul-Hulaifah and I asked him about that. He said: 'I am simply doing that which I saw the Messenger of Allah (ﷺ) doing.'" (DE)
Reference (DE): Sunan an-Nasa'i 1437 (DE)
In-book reference (DE): Book 15, Hadith 5 (DE)
English translation (DE): Vol. 2, Book 15, Hadith 1438 (DE)