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.'" (TR)
Reference (TR): Sunan an-Nasa'i 1437 (TR)
In-book reference (TR): Book 15, Hadith 5 (TR)
English translation (TR): Vol. 2, Book 15, Hadith 1438 (TR)